Editing Repentance Co-Op

<br />{{disambig msg|Repentance's Co-Op mode|the monster|Baby|items that follow Isaac|familiar|Co-op babies|Co-op}}
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In the [[The Binding of Isaac: Repentance|Repentance]] expansion, a new Co-Op mode was added. To access it, when a new run begins, as long as Player 1 does not leave the starting room, up to three additional players may press start. When they do, a small character selection menu will pop-up, and they will spawn in as the chosen character. To use the Co-Op baby mode, Player 1 must leave the starting room.

In '''Repentance Co-Op''', sometimes referred to as True Co-Op mode, all players share the same resource pool of [[coins]], [[bombs]], and [[keys]], but have their own independent [[hearts|health]], [[items]], [[trinket]]s, and [[card]]s/[[pill]]s.

There is currently a beta that is testing '''Online Co-Op''', which allows for up to 4 people to play with each-other, whether they be friends or random people. Besides that, it's the same as the built in Co-Op.

__TOC__

== Changes == 

Certain changes are made to the game whenever multiple player characters are present. They are:

* [[Boss Room]]s drop an extra item for every extra player; when the original item is taken, the other items will spawn afterward.
** Any item pedestals spawned in Boss Rooms will have this mechanic; replacement items will always be from the [[Boss (Item Pool)|Boss Room item pool]], regardless of how they were spawned.
** If the item is destroyed (such as via {{i|Abyss}}, {{i|Eternal D6}}, {{c|Tainted Cain}}, {{i|Void}} etc.), no extra items will spawn after it.
* Players who die will turn into small ghost babies upon death. These babies cannot fly (unless the character has an inherent flight ability, like [[The Lost]] or [[Azazel]]), aggro enemies, pick up items, or use any active items, cards, or pills they're holding. They can still flip switches on the ground and shoot tears, although they will deal 0.5 damage. When a Boss Room is cleared, all dead players will respawn with half a heart (depending on what character they are playing).
* All characters will earn [[Completion Marks|completion marks]] on their post-it notes and their associated [[achievements]] after completing their required objectives, regardless of how much they participated.
** There is also a special set of completion marks for Online Co-Op, that are blue for Normal Mode, and Purple for Hard Mode.

== Notes == 

* Enemies will target any players that are alive at random.
* All players can affect [[Devil Room]] chance in all the normal ways, and all players will feel the effects of anything that changes it. Items that increase Devil Room chance will help everyone earn them, but one character taking red heart damage will hurt everyone's chances at earning them.
* If a character has an item/trinket that boosts the value of certain items/pickups, it will only apply to them. For example, only the character that has {{i|BFFS!}} will have stronger familiars, and only the character that has {{t|Bloody Penny}} will spawn hearts from picking up coins.
* All damage players deal to [[bosses]] is reduced by 20%.
* True Co-Op players can also be spawned in at the first room after using {{i|R Key|n}}.

== General Strategy == 

*
* As items, trinkets, and cards/pills aren't shared between characters, players will have to choose who gets any given item. Consider who needs or can make use of certain items more than others when deciding who to give items to.
* If multiple characters touch different items at the same time in a room that offers multiple choices between items (such as [[Angel Room]]s, alt path [[Treasure Room]]s, or [[Boss Rush]]), all the items touched will be taken before they despawn. With proper timing, this can be used to get more value out of these rooms. However, this might not be worth the risk due to varying internet connections causing delay between characters. 
** If one of the items available is much better than the others, it's usually not worth the risk of getting the timing wrong and losing the powerful item.
* Because all coins, bombs, and keys are shared, it usually doesn't matter which player takes them. 
** Take note if one player has a penny trinket (such as {{t|Bloody Penny}} or {{t|Counterfeit Penny}}), since the trinket effect will only trigger for the player who takes the coin.
* Active items can be traded between characters by swapping them with other active items on pedestals. 
** If one character has {{t|Butter!}}, it can be dropped and given to the character who wants to swap items, circumventing the need for another active item altogether.
* Because all dead characters are revived after clearing a Boss Room, more frail characters (like {{c|The Lost}}, {{c|Keeper}}, and {{c|Tainted Jacob}}) become much more powerful, as they can afford to die; the other characters just have to complete the floor with less help if they do.
** [[Familiars]] dead characters have will despawn, making their absence much more impactful; make sure to give them to characters who are more likely to survive. (needs testing).
*** Exceptions to this rule are the red locusts from {{i|Abyss}} which will continue to follow the player’s ghost and {{c|Tainted Lilith}}’s familiar which acts normally with her ghost but still suffering the same penalties as other characters in this form.
** This mechanic also makes devil deals more affordable; a character can spend all their remaining hearts on devil deals, die, and respawn at the end of the next floor, albeit with only half a heart.
** Items/effects that can instantly kill the player (such as {{i|Damocles}}, {{i|Plan C}}, and {{p|Suicide King}}) also become more viable as a result.
* For Remote Play Co-Op, try to stay close together, as any character who wanders past the edge of the screen in a large room will drag the camera with them, putting the other players off-screen and only giving a vague directional indicator for where they are.
** If the game is in windowed mode on PC, the window can be dragged out to greatly increase the game's resolution without triggering the scaling effect, allowing a better view of larger rooms and more room to maneuver.
* Bombs or explosive items like {{i|IPECAC}} or {{i|Dr. Fetus}} should be handled with extreme caution, as the explosions caused by these items will damage other players.
* Curse Rooms can be used by changing which player uses the door, spreading the damage out.
* Unless going for [[The Beast|Beast]], respawn items should be avoided. This is because they will boot all players out of the room if the player holding them dies. For Boss fights, especially ones where you're more likely to die, this resets the boss's health.
** The main exceptions are {{i|Lazarus' Rags}} and {{i|Inner Child}}, as they revive the player in the same room and do not reset the fight.
** The Beast is unique, as respawn items will not boot you out of the boss room during that fight.

== Character Interactions == 

* {{C|Azazel}} will keep his unique Brimstone as a ghost baby, with the damage penalty applied.
* {{C|Lilith}} will be able to use her {{I|Incubus}} for the current room after dying, but for the rest of the floor her baby will be unable to shoot any tears.
* {{C|Keeper}} will cause all [[Devil Room|Devil Deals]] to be paid for in coins instead of heart containers; 1-heart items will cost 15 cents, and 2-heart items will cost 30 cents. This can be beneficial or harmful depending on the amount of money available and if other characters want to spend their heart containers. He will also maintain his inherent triple shot as a ghost baby.
* {{c|The Forgotten}} will keep his bone club as a ghost baby, with the damage penalty applied. The Forgotten will be unable to toggle between the bone club and normal tears while in this state. (needs testing)
* {{C|Bethany}} will add her unique soul charge counter to the other consumable counters. It will only increase when Bethany gains soul hearts, and the soul charges can only be used by Bethany.
* {{C|Jacob and Esau}} will each have ghost babies after death, Esau's ghost baby arriving shortly after Jacob's. The brother whose health didn’t run out will get all their health back when they revive. (needs testing)
* {{C|Tainted Magdalene}} will make all enemies have a chance to drop temporary red hearts on death, regardless of who killed them. Tainted Magdalene's “hug” attack still guarantees that they drop. This synergizes quite well with characters like Tainted Eve, Tainted Bethany, Lillith, and Normal Eve.
* {{C|Tainted ???}} will add his unique poop counter to the normal bomb counter as long as at least one player isn't playing as him, and only Tainted ??? can use the poops. Regular bombs will be added back to the pickup pool, and any player may pick up poop drops to refill Tainted ???'s supply. Tainted ??? can spawn poop pickups when any character deals damage, and enemies can drop poop pickups when they die, regardless of the character that killed them.
* {{C|Tainted Eve}} will maintain control of all of her clots after death, but they will suffer the same damage penalty. If all of Tainted Eve's clots are dead, she will enter her unique {{i|Whore of Babylon}}-esque state as normal, along with using Sumptorium like {{i|Mom's Knife}}.
* {{C|Tainted Azazel}} can still make use of his unique brimstone and sneeze abilities, with the damage penalty applied. The sneeze will also be smaller.
* {{C|Tainted Lazarus}} will still swap between forms while dead; If one of Tainted Lazarus' forms dies while the other is alive, he will alternate between being a ghost baby who can't interact with items or gain hearts and a normal character who can.
* {{C|Tainted Keeper}} will cause all items other than “quest items” ({{I|The Polaroid}}, {{I|The Negative}}, {{I|key piece 1}}, {{I|key piece 2}}, {{I|knife piece 1}}, {{I|knife piece 2}}, and {{I|Dad's Note}}) to cost coins to pick up. This also affects Devil Deals, much like they would for Keeper. Additionally, all enemies will drop temporary coins when killed, regardless of the player who killed them. He will maintain his quadruple shot as a ghost baby.
* {{C|Tainted Apollyon}}'s locusts deal full damage as a ghost baby, bypassing the damage penalty.
* {{C|Tainted Forgotten}} will keep their bone club as a ghost baby, with the damage penalty applied. They will also be able to move on their own. (needs testing)
* {{C|Tainted Bethany}} will add her unique blood charge counter to the other consumable counters. It will only increase when Tainted Bethany gains red hearts, and the blood charges can only be used by Tainted Bethany.
* {{C|Tainted Jacob}} will still be targeted by [[Dark Esau]] while he's dead, but as he's already dead, Dark Esau can't hurt him. Dark Esau is also unable to hurt any characters other than other Normal/Tainted Jacobs, with the exception of the rings of fire he creates when he spawns. For more notes, read [[Tainted Jacob#Co-Op Mode|this article]].
* If one of the players play as{{C|???}} the other one also will take a blue fly if he destroys a poop.

== Character pairing strategies ==

Players must carefully consider what characters they use together. Some characters have items or mechanics that can help/hurt other characters, and some characters may have to compete for certain resources depending on how they play. 
* In general, characters shouldn't be paired together if they share a weakness. These weaknesses include:
** Health. This can be due to the fact that they:
*** Have poor starting health ({{c|Judas}} and {{c|Azazel}})
*** Are only able to use [[Hearts|soul hearts]] for health ({{c|???}}, {{c|The Soul}}, {{c|Tainted Judas}}, {{c|Tainted ???}}, {{c|Tainted Forgotten}}, and {{c|Tainted Bethany}})
*** Want to spend their health to gain advantages ({{c|Eve}}, {{c|Samson}}, {{c|Lazarus}}, {{c|Lilith}}, and {{c|Tainted Eve}})
**** Both forms of {{c|The Lost}} and {{c|Keeper}} have their own unique forms of health and won't have to compete for health.
** Speed ({{c|Magdalene}}, {{c|Keeper}}, {{c|Tainted ???}}, and {{c|Tainted Lilith}})
** Range ({{c|Cain}}, {{c|Samson}}, {{c|Lazarus}}, {{c|Tainted Judas}}, {{c|Tainted Samson}}, and {{c|Tainted Lazarus}})
** Starting Damage ({{c|Eve}}, {{c|Lilith}}, {{c|Tainted ???}}, {{c|Tainted Eve}}, and {{c|Tainted Apollyon}})
*** {{c|Eden}}/{{c|Tainted Eden}} can have any combination of these weaknesses, but health and damage are the most important and common for them.
* Characters with flight will be able to collect pickups behind rocks, spikes, and pits so other characters don't have to spend resources getting them.
* More frail characters (like {{c|The Lost}}, {{c|Keeper}}, and {{c|Tainted Jacob}}) will respawn after each boss fight, mitigating that weakness.


=== {{c|Isaac}} ===

; Strengths
: {{i|The D6}} (if unlocked)
: No true weaknesses
; Weaknesses
: None

* Isaac's D6 should be used before any other effects that interact with items, such as the {{i|Eternal D6}}, {{i|Void}}, or {{i|Abyss}} (which are held by {{c|The Lost}}, {{c|Apollyon}}, and {{c|Tainted Apollyon}}, respectively), as destroyed items cannot be rerolled.
** This is especially true for Boss Room items: If the item is destroyed, no extra items will spawn after it, reducing the amount of items gained overall.

Isaac generally works well with any character, due to his neutral stats and variable game plan. If {{I|The D6}} start is unlocked, stacking multiple Isaac players can be effective to improve the general quality of items picked up, as each item can be rerolled more times, or multiple boss pedestal items can be rerolled before being picked up. Isaac's D6 can also be used in place of the {{I|Eternal D6}} to remove the danger of deleting an item altogether.

=== {{c|Magdalene}} ===

; Strengths
: High starting health 
: {{i|Yum Heart}}
: A Full Health pill (if unlocked)
; Weaknesses
: Low speed

* The Full Health pill will be identified for all characters, letting everyone make use of them.
* Yum Heart and the Full Health pill can be given to other characters. This is especially useful for the following characters:
** {{c|Samson}} and {{c|Lilith}} can use the extra health to get better use out of {{i|Bloody Lust}} and {{i|Cambion Conception}} respectively.
** {{c|Bethany}} can get good use out of Yum Heart due to its interaction with {{i|Book of Virtues}} and the fact that she can only have red hearts for health. 
** {{c|Tainted Eve}} can directly convert the extra health into damage in the form of blood clot familiars.
** {{c|Tainted Bethany}} can use Yum Heart to give herself extra blood charges to use {{i|Lemegeton}}. However, the full health pill won't affect her.
* The large amount of hearts Magdalene can generate gives her and her allies lots of hearts to spend on [[Curse Room]]s, [[Sacrifice Room]]s, [[Machines#Blood Donation Machine|Blood Donation Machines]], [[Beggar#Devil Beggar|Devil Beggars]], [[Machines#Confessional|Confessionals]], and [[Beggar#Hell Game|Hell Games]].
** Characters that can't use red hearts (such as {{c|???}}, {{c|The Lost}}, and {{c|Keeper}}) won't be able to make use of the extra health, but it won't stop Magdalene from making use of the extra health herself.

Magdalene's main benefit as a co-op partner comes in the form of her {{i|Yum Heart}} and full health pill (if unlocked). The health these provide can still prove to be a valuable resource for using [[Machines#Blood Donation Machine|Blood Donation Machines]], [[Beggar#Devil Beggar|Devil Beggars]], and [[Sacrifice Room|Sacrifice Rooms.]]The fact that the full health pill will be identified for all characters is especially helpful when the pill spawns in the future. Her high starting health also means she won't be taking other characters' health ups, but her low starting speed means she'll want the first speed up that spawns more than the others.

For characters that pair well with Magdalene, {{c|Samson}} and {{c|Lilith}} can use the extra health to get better use out of {{i|Bloody Lust}} and {{i|Cambion Conception}} respectively, {{c|Bethany}} can get good use out of Yum Heart due to the fact that she can only have red hearts for health, and {{c|Tainted Eve}} can directly convert the extra health into damage in the form of blood clot familiars. Characters that can't use red hearts (such as {{c|???}}, {{c|The Lost}}, and {{c|Keeper}}) won't be able to make use of the extra health, but this doesn't really detract from Magdalene's personal strengths. The only characters that are highly advised to not pair with Magdalene are {{c|Keeper}}, {{c|Tainted ???}}, and {{c|Tainted Lilith}}, as they also have low speed and will have to compete with Magdalene for speed upgrades.

=== {{c|Cain}} ===

; Strengths
: Good starting speed and damage
: {{i|Lucky Foot}}
: {{t|Paper Clip}} (if unlocked)
; Weaknesses
: Low range

* Cain's Lucky Foot and Paper Clip help generate resources in the early game that everyone can use.
* Cain's Lucky Foot lets him safely take pills. While other characters won't have their pills converted, pills Cain uses will still be identified for everyone else in the future.
** {{c|Lazarus}} starts with a pill that Cain can use.
* If players want to gamble, Cain should be the one who does so, as only he gets the gambling bonus from Lucky Foot.
* {{i|Technology 2}} should be given to characters other than Cain, as he won't be able to fire any normal tears due to having one eye. {{I|Chemical Peel}} and {{I|Blood Clot}} are marginally less effective on him as well, because of his single eye.

Cain has a generous array of benefits by default. His {{T|Paper Clip}} starting item, if unlocked, can massively improve the entire team's item economy, allowing other players to more freely use keys for rooms like the shop. His inherent {{I|Lucky Foot}} also means he can take pills spawned by other players, or even the pill that {{C|Lazarus}} starts with to force a neutral or good effect for himself. 

His biggest weakness is a lack of range and the interactions caused by missing one eye, so items like {{I|Technology 2}} should always be given to others, while reserving range increases like {{I|Mom's Heels}} for him will show massive benefits.

=== {{c|Judas}} ===

; Strengths
: High starting damage
: {{i|The Book of Belial}}
; Weaknesses
: Very low starting health

* Judas' Book of Belial will make [[Library|Libraries]] more common, making getting the [[Bookworm]] transformation much more likely for all players.
Judas provides a high damage output from early on, and won't be competing for an active item. However, his extremely low HP will put him at odds with a few characters, as he will either be taking multiple HP ups or trying to gather as many Soul or Black Hearts as possible, making him a poor companion to characters like {{C|Azazel}} or {{C|???}}.

=== {{c|???}} ===

; Strengths
: Slightly increased damage and speed
: Will never take red hearts
; Weaknesses
: Can only have soul hearts for health

* {{c|Bethany}} can be given {{i|The Poop}} to create a steady stream of wisps that can turn enemies into [[Poops]], similarly to {{i|E. Coli}}.
* ??? is one of few characters with next to no reasons to be used as a co-op partner aside from earning achievements or giving yourself a challenge.

=== {{C|Eve}} ===

; Strengths
: High speed
: {{i|Whore of Babylon}} (and {{i|Razor Blade}} to use it, if unlocked)
; Weaknesses
: Will want soul hearts more than other characters, as she wants to be at low health constantly
: Low damage without Whore of Babylon active

* Eve will want to use Devil Rooms instead of Angel Rooms to spend her heart containers, putting her at odds with {{c|Bethany}}, {{c|Tainted Magdalene}}, and other low-health characters.
** {{c|Keeper}} and {{c|Tainted Keeper}} make Devil Rooms cost coins instead of hearts, preventing Eve from using them effectively.

Eve can be somewhat of a high maintenance character to play with. Players that prioritize staying in her {{I|Whore of Babylon}} state will need to either get rid of heart containers, or avoid sources of health, both while collecting Soul or Black Hearts. If not, she will need more damage ups than usual to overcome her inherently lower damage multiplier to remain effective across floors. Despite these difficulties, Eve can pair well with Samson.

=== {{C|Samson}} ===

; Strengths
: {{i|Bloody Lust}}
: {{t|Child's Heart}} (if unlocked)
; Weaknesses
: Low range
: Will want hearts more than other characters

* {{c|Magdalene}}'s {{i|Yum Heart}} and Full health pill and {{c|Tainted Magdalene}}'s ability to make enemies drop hearts on death can help Samson activate Bloody Lust more easily.
* Samson will want on-hit items/trinkets more than other characters, as he'll be taking damage frequently to use Bloody Lust, with the exception of Tainted Magdalene (if she's being played offensively).
* Samson will have to compete with {{c|Lilith}} and {{c|Tainted Magdalene}} for on-hit items, and Lilith, {{c|Tainted Eve}}, and {{c|Tainted Bethany}} for hearts to use.

Samson pairs well with any character that does not need to pick up red hearts, such as ???, Azazel, or Eve. Magdalene is also another easy choice thanks to her Yum Heart starting item allowing him to always take an extra hit to build up his {{I|Bloody Lust}} bonus. If played well, Samson can get away with less damage ups than other characters in Co-Op, as long as his HP or Tear stats are given priority.

=== {{C|Azazel}} ===

; Strengths
: Highest starting damage
: High speed
: Flight
; Weaknesses
: Very low range
: No starting heart containers

* Azazel will want tear modifiers more than the other characters to negate his poor range.

Azazel's inherent flight and high damage makes him a strong choice to play with. Because players can decide whether or not he will be using red heart containers, he can be used with characters that have specific health demands such as Eve, ???, or Judas. His higher damage means that other characters should get the chance to pick up a damage increasing item first, as range is Azazel's main problem early on.

=== {{C|Lazarus}} ===

; Strengths
: Good speed and damage after reviving
: {{i|Anemic}} (which will always be active after reviving if [[Backasswards]] has been cleared)
; Weaknesses
: Low range and luck before reviving
: Very low health right after reviving

* Lazarus starts with a pill that {{c|Cain}} can use.

Lazarus suffers mainly from his below average range and his bad luck stat. Other than that, he's a somewhat neutral character. If his {{I|Anemic}} start is unlocked, his range will be less of an issue, and he can play recklessly to use creep to damage enemies. Lazarus serves as somewhat of a "clutch" character, being able to revive immediately after dying while remaining in the room in his Lazarus Risen form and improving several of his stats, including his damage multiplier. 

=== {{C|Eden}} ===

; Strengths
: Random starts can be very strong.
; Weaknesses
: Random starts can be very weak.

* Eden's random generation can give them any list of strengths and weaknesses, but health and damage have the greatest potential to cause problems.

Because Eden has entirely random stats and items, there's no consistent strategy for playing with them, since each run needs to be adapted to their needs during that specific attempt. The best overall strategy for playing with Eden is bringing a much more consistent, stable character such as Isaac that does not need incredible maintenance or has some flexibility to allow for Eden's mutations safely.

=== {{C|The Lost}} ===

; Strengths
: Flight
: {{i|Eternal D6}}
: {{i|Holy Mantle}} (if unlocked)
: Can take one Devil Room item for free per floor
: Does not have to compete with other characters for health
; Weaknesses
: Cannot take damage without dying

* The Lost should be the last one to buy something from a Devil Room: all other options will disappear after he takes an item.
* The Lost's Holy Mantle lets him enter and exit Curse Rooms without spending health.
* The Lost's Eternal D6 should be used after {{c|Isaac}}'s normal {{i|The D6}} due to its potential to destroy items; whether or not it should be used before {{i|Void}}/{{i|Abyss}} (held by {{c|Apollyon}}/{{c|Tainted Apollyon}} respectively) should be judged on an item-by-item basis.
** This is especially true for Boss Room items: If the item is destroyed, no extra items will spawn after it, reducing the amount of items gained overall.

If The Lost has their Holy Mantle start unlocked, they can be a surprisingly good partner for Co-Op. Since The Lost does not need to take any on-hit items or HP ups, they immediately become compatible with many different characters like Judas, Samson, Eve, or even ???, though these runs pose an obvious risk early on. However, playing with The Lost also makes items such as {{I|IPECAC}} extremely dangerous to take due to the risk of friendly fire. Because of the ability for Dark Esau to deal damage to all players when he spawns, and the overlap in Devil Deal uses, The Lost pairs ''very'' poorly with {{C|Tainted Jacob}}.

If multiple players choose The Lost, they can pick up devil deal items at the exact same time and gain multiple free devil deals across the team. An easy and consistent way of doing this is do pause the game while all players are against the bottom wall holding upwards; upon unpausing all players will begin to float upwards at the exact same time.

=== {{C|Lilith}} ===

; Strengths
: {{i|Cambion Conception}} and {{i|Box of Friends}}
; Weaknesses
: Blindfolded: must rely on {{i|Incubus}} to deal damage, disjointing her hitbox and where she fires tears from
: Only ghost baby that absolutely cannot shoot
: Only one starting heart container

* {{c|Magdalene}}'s {{i|Yum Heart}} and Full health pill and {{c|Tainted Magdalene}}'s ability to make enemies drop hearts on death can help Lilith activate Cambion Conception more easily.
* Lilith will want familiars, {{i|BFFS!}}, and {{t|Forgotten Lullaby}} more than other characters, due to her familiar-based setup.
* Lilith will want on-hit items/trinkets more than other characters, as she'll be taking damage frequently to use Cambion Conception, with the exception of Tainted Magdalene (if she's being played offensively).
** Lilith will also want the first health up more than other characters so she can make better use of red hearts to do this.
* Lilith will have to compete with {{c|Samson}} and {{c|Tainted Magdalene}} for on-hit items, Samson, {{c|Tainted Eve}}, and {{c|Tainted Bethany}} for hearts to use, and Tainted Eve and {{c|Tainted Apollyon}} for familiar-boosting items.

Lilith suffers from her unusual starting health, meaning she must procure either Soul/Black Hearts, or stick to slowly getting HP ups to stay alive. However, because of her exponential strength with {{I|Box of Friends}} she can be very strong when kept alive. Lilith pairs well with Magdalene thanks to {{I|Yum Heart}} allowing her to take more damage to activate {{I|Cambion Conception}} more often, as well as any character that can protect her. Because of her base health, players should avoid pairing her with ??? or {{C|Tainted Judas}} because of their health needs.

Lilith ''cannot'' clone familiars owned by other players, so items that grant her something that could be cloned by Box of Friends should be given to her immediately.

=== {{C|Keeper}} ===

; Strengths
: Heals with coins instead of hearts
: Only needs one HP Up item (or none if Hush has been defeated with Keeper)
: Good damage and intrinsic {{i|The Inner Eye}}
: {{i|Wooden Nickel}} (if unlocked)
: {{T|Store Key}} (if unlocked)
: Cannot decrease Devil Room chance by taking damage 
; Weaknesses
: Max 3 health (2 if [[Hush]] has not been defeated with Keeper)
: Low speed
: Very low tears
: Very low luck

* Devil Rooms will cost coins instead of hearts, which is situationally better/worse depending on the amount of money available and whether or not characters want to spend their heart containers.
** {{c|Eve}} will want to spend her heart containers in Devil Rooms, making this almost strictly worse for her.
* Keeper can use Curse Rooms and Sacrifice Rooms at the cost of coins instead of other players' more valuable hearts.
** The 10th use of a Sacrifice Room will give 30 coins (which pays back the cost of using it) or 7 soul hearts (which other characters can use, and are much more valuable than 10 cents).
** Everyone must leave behind coins for Keeper to backtrack for and use in order for him to use this strategy, making it ineffective if [[Boss Rush]] and/or [[Hush]] are priorities on the current run.
* If unlocked, Keeper can use {{T|Store Key}} as a starting item, meaning players who want to spend money can have Keeper open the door for them.
* Because Keeper's low tears is due to a low tears value instead of a tear delay multiplier, Keeper can make good use of excess tears upgrades.
* Other characters should take pills and {{i|Experimental Treatment}} instead of Keeper if there aren't any health ups on the floor, due to the risk of health downs.
* Other characters should take {{i|Piggy Bank}} and {{t|Swallowed Penny}} instead of Keeper; they spawn less coins if used by him.
* {{c|Bethany}} can be given Wooden Nickel to create wisps that also spawn extra coins.

Keeper's unique health situation revolving around coins can actually be very useful in a Co-Op situation, as he will never require ''any'' hearts that another player might need. Soul Hearts will also not be consumed immediately. If another player acquires {{I|Piggy Bank}} without playing Keeper, they will have normal coin drops from it, keeping him alive much longer than normal. Keeper will also make Devil Deals cost pennies instead of heart containers, which while a bit of a detriment to characters like Eve, can be useful generally for other characters such as ???. 

Keeper needs to take items that increase his rate of fire, or speed, but depending on the host's progress in their save file will either need only one HP up or none at all.

=== {{C|Apollyon}} ===

; Strengths
: {{i|Void}}
; Weaknesses
: Low health

* Apollyon's Void should be used after {{c|Isaac}}'s {{i|The D6}} due to the fact that it destroys the item, preventing it from being rerolled; whether or not it should be used before or after {{c|The Lost}}'s {{i|Eternal D6}} or in place of {{c|Tainted Apollyon}}'s {{i|Abyss}} should be judged on an item-by-item basis.
** This is especially true for Boss Room items: If the item is destroyed, no extra items will spawn after it, reducing the amount of items gained overall.
* Because of how Void interacts with items and the fact that it can be given to other characters, there are many intricacies to be understood when using it effectively in co-op:
** Unlike {{i|The D6}} where any character can get the rerolled item, only the user of Void gets stat-ups from destroying passive items.
*** If one character is lacking in stat upgrades, Void can be given to that character to help them fix their stats.
** All active items destroyed by Void will activate each time Void is used, regardless of the character that destroyed the items, and their effects will only apply to the character that uses Void.
*** If Void destroys active items that give health or persistent bonuses (such as {{i|Book of Revelations}}, {{i|Smelter}}, {{i|Lemegeton}}, etc.), it should be given to characters who need the effects of the absorbed items more. 
**** Don't forget that active items destroyed by Void won't give stats, and Void has a 6-room recharge; judge whether or not it's better to use Void's charge on stacking active item effects or giving the user stat upgrades.
**** {{c|Bethany}} will also spawn the wisp of every active item destroyed by Void each time she uses it; whether or not she should use a single item with a shorter cooldown or be given Void to summon multiple wisps each time she uses it should be determined on an item-by-item basis.

Apollyon is a relatively neutral partner for Co-Op, due to {{I|Void}} being usable in the same situations as which it normally would be. Be wary as using Void on a Boss Pedestal will destroy all items, but only give the benefit of the one consumed. Apollyon starts with below average health, but other than that has no notable item requirements. Apollyon's main benefit is that he can absorb undesirable items while still getting minor benefits from it. It is often a bad idea for one player to play as Apollyon while another plays as {{C|Isaac}}, {{C|Apollyon}}/{{C|Tainted Apollyon}}, or {{C|Tainted Cain}}, as their special abilities can make Apollyon's {{I|Void}} irrelevant or risky.

Apollyon's synergizes particularly well with {{C|Tainted Isaac}} as Apollyon can absorb disposed items and gain stat boosts instead of wastefully leaving them on the pedestal.

=== {{C|The Forgotten}}/{{c|The Soul}} ===

; Strengths (as The Forgotten)
: High damage
: Can open [[Chests#Spiked Chest|spiked chests and mimics]] for free w/ bone club
: Can be used as a bullet shield when The Soul is in control
; Strengths (as The Soul)
: Flight
; Weaknesses (as The Forgotten)
: Can only have red/bone hearts for health
: Very low range
; Weaknesses (as The Soul)
: Can only have soul hearts for health
: Very low starting health
: Limited mobility

* {{c|Bethany}} will have to compete with The Forgotten for health, as she is also only able to use heart containers/bone hearts for health.
* Size ups will increase the range of The Forgotten's bone club, and have higher priority for him.
** Inversely, size downs will decrease the club's range, and have lower priority for him.
* The Forgotten's interactions with tear modifiers should be considered when choosing who to give them to.

The Forgotten is a very high maintenance character in solo play, and that is reflected in their Co-Op performance. However, the high damage output of the bone club and the ability to use the club or The Soul to collect pick-ups easily is a massive boon for players as they share keys, bombs, and pennies. The Forgotten can also send out The Soul and allow at least one other player to stand behind them as a shield, offering some protection. Their melee based playstyle often puts them near enemies, meaning more fragile characters such as {{C|Judas}} may be better with them to avoid taking damage.

=== {{C|Bethany}} ===

; Strengths
: {{i|Book of Virtues}}
:: The book guarantees the first Devil Room will be an Angel Room instead
: Can use soul hearts to use her active item more often
; Weaknesses
: Cannot use soul hearts for health
:: Cannot protect Devil Room chance unless she has a bone heart

* Soul hearts are generally more useful as health than active item charges, restricting the amount of Soul Hearts she can use for her active item.
** If the other characters can't use soul hearts (both forms of {{c|The Lost}} and {{c|Keeper}}, plus {{c|Tainted Magdalene}}), Bethany is free to use soul hearts for her active item.
* Active items will need more consideration than normal when choosing who to give them to, given how they can act with Book of Virtues.
** Characters whose active items don't tie directly into their game plan can give their active item to Bethany, as Book of Virtues will make the item better.
* {{c|The Forgotten}} will have to compete with Bethany for health, as they are also only able to use heart containers/bone hearts for health.

Bethany is not necessarily high maintenance, but she is very unique and has her drawbacks as a partner. Because of her soul charge mechanic, she will likely want to collect soul hearts and black hearts, which puts her at odds with characters like ???, Tainted Judas, or Azazel. Characters who prefer taking Devil Deals such as Tainted Jacob, The Lost, or The Forgotten may also be alienated due to her increased ability to acquire Angel Rooms with the {{I|Book of Virtues}}. Still, when allowed to collect these hearts and given some extra protection from another, Bethany's explosive potential is easily exploited.

=== {{C|Jacob and Esau}} ===

; Strengths
: High starting damage
: Has two starting health pools
; Weaknesses
: Greatly decreased ability to dodge, due to having two hitboxes
: Items and health will have to be split between two characters
:: Speed upgrades will be half as effective

* Jacob and Esau have increased control over the camera, due to the fact that they count as two characters.
* Jacob and Esau are two of few characters with next to no reasons to be used as co-op partners aside from earning achievements or giving yourself a challenge.

Jacob and Esau are difficult and sometimes unwieldy to play in solo, even more so in Co-Op. Due to the fact that they always spawn multiple characters, the clutter on-screen is always more than it would normally be thanks to the presence of Esau. The very specific, and needy, nature of their item selection process can be frustrating for other players to deal with in order to ensure they stay alive. The general difficulty of playing with them is increased in Co-Op, making them unfavorable to use.

=== {{C|Tainted Isaac}} ===

; Strengths
: All new items reroll between two items to choose from
: Able to give unneeded items to other players when inventory is full.
; Weaknesses
: Can only hold 8 passive items at a time

* The 8-item limit doesn't apply to other characters, letting them take full advantage of Tainted Isaac's unique reroll mechanic.
* If Tainted Isaac is at his 8-item limit, he can effectively drop his passive items by picking up more items, letting other characters have them instead.

Tainted Isaac is a high potential co-op partner, as his more options benefit works for all players. 

* Paired with {{C|Isaac}}, you can cycle through 4 different choices with a single {{I|The D6}} charge, giving substantially higher odds of finding good items across a run.
* Tainted Isaac's limited inventory size rarely comes into play, as all items will usually be split between multiple players. If Tainted Isaac ends up needing to put down an item, the other players can simply take the item and retain the benefits for the team.
* Tainted Isaac can take advantage of[[Tainted Lost|{{c|Tainted Lost}}]]'s "better items" ability in combination with his free rerolls to increase the chance of finding higher quality items. Although Tainted Lost removes health ups from item pools, meaning there are less ways for other players to gain heart containers, the potential to become very powerful may offset that weakness.

=== {{C|Tainted Magdalene}} ===

; Strengths
: Has a “hug” attack that deals 6x her damage to enemies that touch her
: Enemies have a chance to drop fading half red hearts on death. This applies to all players, even if they are not Tainted Magdalene.
:: This is guaranteed if they were killed by Tainted Magdalene's “hugs”
: {{i|Yum Heart}}, which heals Tainted Magdalene for twice as much
: Taking red heart damage does not decrease Devil Room chance if she has more than 2 red hearts left
: Usually doesn't use Soul Hearts, allowing other characters who may need them. See weaknesses.
; Weaknesses
: Health drains over time
: Soul hearts are almost immediately lost, either due to dealing contact damage to enemies or draining over time
: Cannot go below 2 red hearts without risking Devil Room chance and survivability
: Low damage if not dealing contact damage

* Hearts dropped by enemies can be used to heal Tainted Magdalene's allies; the health granted by this will not drain over time.
** {{c|Samson}} and {{c|Lilith}} can get better use out of {{i|Bloody Lust}} and {{i|Cambion Conception}} respectively, {{c|Tainted Eve}} can convert the extra health into damage in the form of blood clot familiars, and {{c|Tainted Bethany}} can turn excess hearts into blood charges for her active item(s).
** If Tainted Magdalene is played this way, she also sacrifices most of her offensive potential. If not, she will have to compete with Samson and Lilith for on-hit items/trinkets.
** Unlike normal {{c|Magdalene}}, her Yum Heart can't be traded to other characters, as it is in her consumable slot.
* The large amount of hearts Tainted Magdalene can generate gives her and her allies lots of hearts to spend on [[Curse Room]]s, [[Sacrifice Room]]s, [[Machines#Blood Donation Machine|Blood Donation Machines]], [[Beggar#Devil Beggar|Devil Beggars]], [[Machines#Confessional|Confessionals]], and [[Beggar#Hell Game|Hell Games]].
* Size ups will increase the range of Tainted Magdalene's “hugs”, and have higher priority for her.
** Inversely, size downs will decrease the “hugs”' range, and have lower priority for her.
* Tainted Magdalene will want shield/invulnerability effects more than other characters to use her “hugs” against bosses, who won't be killed in only 1-2 “hugs”.
* Characters that use only soul hearts for health won't have to compete for them, as Tainted Magdalene can only really use red hearts.
* Characters that can't use red hearts (such as {{c|???}}, {{c|The Lost}}, and {{c|Keeper}}) won't be able to make use of the extra health, but it won't stop Tainted Magdalene from making use of the extra health herself.

Tainted Magdalene is able to function two different ways; an offensive, face-tanking damage dealer or heart-generating support character.

As an offensive character, Tainted Magdalene is able to run into enemies constantly to deal high damage, and still retain her health due to all enemies dropping half hearts when killed this way. It also lets her make make incredible use of items that activate upon taking damage, even more so than characters like {{c|Samson}} and {{c|Lilith}} because she'll always be generating the health to keep taking damage. The main drawbacks to this strategy are that Devil Rooms will be off limits because Tainted Magdalene can't function without having a good supply of heart containers, and if her red heart buffer runs out, she won't be able to play aggressively without costing her partners' Devil/Angel room chance.

As a supporting character, Tainted Magdalene forgoes using the hearts dropped by enemies to deal damage in favor of letting her allies pick up the hearts themselves. Due to the fact that her allies' health doesn't drain over time, their health can easily be kept filled. In the cases of {{c|Samson}}, {{c|Lilith}}, and {{c|Tainted Eve}}, this can instead be used to greatly increase their damage potential; {{i|Bloody Lust}} can be maxed out in only a few rooms, all four of {{i|Cambion Conception}}'s familiars can be obtained in a few floors, and {{i|Sumptorium}} will be able to generate a never-ending army of clot familiars. This strategy comes at the cost of Tainted Magdalene herself being very weak; she'll have to use her much weaker tears instead, rely on her {{i|Yum Heart}} to sustain herself, and be unable to protect her team's Devil Room chance.

=== {{C|Tainted Cain}} ===

; Strengths
: High speed
: Can convert weak/useless/unknown items into pickups
:: One of these pickups will always be a [[Hearts|non-red heart]], depending on the room the item was in
: Can convert pickups other characters aren't using into items he can use
; Weaknesses
: Can only gain items through {{i|Bag of Crafting}}
: Players have to be very alert to not pick up items Tainted Cain may want 

* If Tainted Cain converts a boss item into pickups, no other items will spawn after it.
* Tainted Cain isn't able to pick up multiple-choice items alongside other characters, due to the fact that he doesn't actually pick up the item. (needs testing)
* Due to the fact that most items are worth taking instead of converting into pickups, Tainted Cain will have a decreased supply of pickups to turn into items, mostly making him a support character that makes bad items more useful.
* Overall, don't expect to play any matches with him online, as players will likely kick you.

=== {{C|Tainted Judas}} ===

; Strengths
: {{i|Dark Arts}} gives high damage, enemy freezing, bullet stopping, and free Curse Rooms all on command.
: High speed
: High damage output
; Weaknesses
: Low range
: Can only have soul hearts for health

* Dark Arts can be used to cross spikes and enter and exit Curse Rooms without spending health.
* Dark Arts can be used to protect himself and others near him from bullet blasts.

=== {{C|Tainted ???}} ===

; Strengths
: Uses poops instead of bombs, which are independent from other characters' bomb supply
: High damage potential from poops, which reduces his need for damage ups
; Weaknesses
: Can only have soul hearts for health
: Low speed and tears

* Tainted ??? spawns poop pickups when ''any'' character deals damage, not just him, and enemies can drop poop pickups regardless of the character that killed them.
** Poop pickups will not replace bomb pickups in co-op if there are characters other than Tainted ???.
* The damage of Tainted ???'s poop is independent of Tainted ???'s damage and tears, with the exception of Blue Flies from corny poop and the damage enemies take from walking over creep. Combined with the previous note, Tainted ??? has the unique distinction of not requiring ''ANY'' damage upgrades of any kind for his damage potential to scale into the late game; only his partners need damage upgrades.
* Tainted ???'s poops are able to create explosions under the right circumstances, which lets him blow up [[Rocks#Tinted Rocks|Tinted Rocks]] and [[Secret Room]] doors and lets other characters save their bombs for other uses.
** Caution should be taken when trying to use poops offensively; these explosions can hurt other characters.
* Creep and White Poops created by Tainted ??? can buff all other characters in range, greatly boosting the damage of all of Tainted ???'s co-op partners.
** White Poop gives its benefits even while Tainted ???'s holding it over his head, allowing him to become a mobile damage multiplier for other characters, similar to {{i|Succubus}}. The poop will be dropped if he takes damage, however.

=== {{C|Tainted Eve}} ===

; Strengths
: Can convert hearts directly into damage in the form of clot familiars
: If all clots are killed, has a (relatively) extremely damaging ghost baby that wields {{I|Sumptorium}}
; Weaknesses
: Enormous heart demands for hearts of all kinds, due to wanting to keep creating clots
: Has the lowest damage of all characters w/o her clot familiars

* {{c|Magdalene}}'s {{i|Yum Heart}} and Full health pill and {{c|Tainted Magdalene}}'s ability to make enemies drop hearts on death can help Tainted Eve spawn more clot familiars.
* Tainted Eve will want {{i|BFFS!}} and {{t|Forgotten Lullaby}} more than other characters, due to her familiar-based setup.
* Tainted Eve will have to compete with {{c|Samson}}, {{c|Lilith}}, and {{c|Tainted Bethany}} for hearts to use and Lilith and {{c|Tainted Apollyon}} for familiar-boosting items.
* Tainted Eve might also have to compete with {{C|???}} or {{C|Tainted Judas}} for other types of hearts.
* Unlike normal Eve, Tainted Eve works greatly with both Keepers, as they only use coins for hearts, meaning Eve can have all the hearts to herself.

=== {{C|Tainted Samson}} ===

; Strengths
: Activates {{i|Berserk!}} after taking/dealing enough damage, greatly increasing his damage for a short period of time, especially helpful in earlygame.
: Can activate Berserk! as a ghost baby for increased damage potential when dead.
; Weaknesses
: Low range
: Inconsistent damage

=== {{C|Tainted Azazel}} ===

; Strengths
: Infinite range
; Weaknesses
: No starting heart containers
: Deals half damage to enemies unless he uses {{i|Hemoptysis}} on them first

Tainted Azazel synergizes quite well with his normal counterpart due to Hemoptysis boosting the damage of any Brimstone attack, not just his own.

=== {{C|Tainted Lazarus}} ===

; Strengths
: Has two starting health pools
; Weaknesses
: Items must be split between two characters, of which only one will be active at any given time without Birthright.

* Tainted Lazarus will still swap forms while dead; If one of Tainted Lazarus' forms dies while the other is alive, he will alternate between being a ghost baby and a normal character.
** This allows his item limitation to be handled one of two ways: divide items between the two characters and play them normally, or let one character have all the items and let the other have none, making Tainted Lazarus a character that's only active in every other room.
* Tainted Lazarus is one of few characters with next to no reasons to be used as a co-op partner aside from earning achievements or giving yourself a challenge.

=== {{c|Tainted Eden}} ===

; Strengths
: Everything about Tainted Eden gets rerolled after they take damage.
; Weaknesses
: '''Everything''' about Tainted Eden gets rerolled after they take damage.

* Weak/useless items, trinkets, and cards/pills can be given to Tainted Eden with no consequence; they will turn into other items after they take damage.
** The only exception is Devil/Curse room items, as they can turn into {{i|Dead Cat}} and remove all of Tainted Eden's health with no benefit.
* If Tainted Eden rerolls into a good active item/trinket/card/pill, it can be given to another character so they can make full use of it without having to worry about it getting rerolled.
** This can be forced by letting Tainted Eden spend their health in place of other characters. Note that Tainted Eden's health can fluctuate drastically after they take damage.

As with normal Eden, the best partner to use in Co-Op with them is any character that is very stable or low maintenance, such as {{C|Isaac}}

=== {{C|Tainted Lost}} ===

; Strengths
: High damage
: Flight
: Can take one Devil Room item for free per floor
: Does not have to compete with other characters for health
: Prevents most weak items from spawning
; Weaknesses
: Cannot take damage without dying
: Replaces 10% of card and rune spawns with {{p|Holy Card}}, slightly reducing the number of cards/runes available to use

* As most items that cannot spawn are health ups or items that activate upon taking damage, {{c|Samson}}, {{c|Lilith}}, {{c|Tainted Magdalene}}, and {{c|Tainted Eve}} will be much less able to take advantage of their strengths.
* Other characters can hold onto a Holy Card for Tainted Lost for the late game or when health starts becoming a problem.
* A lack of inherent Holy Mantle means that playing with {{C|Tainted Jacob}} runs the risk ''every floor'' that Dark Esau's spawning animation will kill him. This, in addition to all of the problems of playing Tainted Jacob with {{C|The Lost}} makes him incredibly bad as a Co-Op partner.

=== {{C|Tainted Lilith}} ===

; Strengths
: High starting damage
: Fixed range
: Keeps her familiar in death
; Weaknesses
: Low speed
: Only one starting heart container
: Unable to interact as effectively with most tear modifiers

* Tainted Lilith's fixed range lets low-range characters ({{c|Cain}}, {{c|Samson}}, {{c|Lazarus}}, {{c|Tainted Judas}}, {{c|Tainted Samson}}, and {{c|Tainted Lazarus}}) take all range ups without having to worry about sharing them.
* Tainted Lilith will want tear modifiers less than other characters, due to the fact that most of them don’t interact well with her unique form of attack.

=== {{C|Tainted Keeper}} ===

; Strengths
: Heals with coins instead of hearts
: Intrinsic {{i|Mutant Spider}}
: Cannot decrease Devil Room chance by taking damage 
: All enemies drop fading coins when killed
:: Enemies spawned by other enemies only have a chance to drop coins, depending on how many of them have been spawned
: Shops have a greatly increased stock of items/pickups available, with added [[Treasure Room]] and [[Boss Room]] items
; Weaknesses
: Max 2 health
: Very low tears
: All non-quest items ({{I|The Polaroid}}, {{I|The Negative}}, {{I|key piece 1}}, {{I|key piece 2}}, {{I|knife piece 1}}, {{I|knife piece 2}}, and {{I|Dad's Note}}) are no longer free, and cost money to obtain

* Devil Rooms will cost coins instead of hearts, which is situationally better/worse depending on the amount of money available or the Co-Op partners in play.
** {{c|Eve}} will want to spend her heart containers in Devil Rooms, making this almost strictly worse.
* Tainted Keeper can use Sacrifice Rooms at the cost of coins instead of other players' more valuable hearts.
** The 10th use of a Sacrifice Room will give 30 coins (which pays back the cost of using it) or 7 soul hearts (which other characters can use, and are much more valuable than 10 cents).
** Everyone must leave behind coins for Tainted Keeper to backtrack for and use in order for him to use this strategy, making it ineffective if [[Boss Rush]] and/or [[Hush]] are priorities on the current run.
* Because Tainted Keeper's low tears is due to a low tears value instead of a tear delay multiplier, Keeper can make good use of excess tears upgrades.
* Other characters should take pills and {{i|Experimental Treatment}} instead of Keeper if there aren't any health ups on the floor, due to the risk of health downs.
* Other characters should take {{i|Piggy Bank}} and {{t|Swallowed Penny}} instead of Keeper; they spawn less coins if used by him.

=== {{C|Tainted Apollyon}} ===

; Strengths
: {{i|Abyss}}
: Locusts persist after death
; Weaknesses
: Low tears

* Tainted Apollyon's Abyss should be used after {{C|Isaac}}'s {{i|The D6}} due to the fact that it destroys the item, preventing it from being rerolled; whether or not it should be used before or after {{c|The Lost}}'s {{i|Eternal D6}} or in place of {{c|Apollyon}}'s {{i|Void}} should be judged on an item-by-item basis.
** This is especially true for Boss Room items: If the item is destroyed, no extra items will spawn after it, reducing the amount of items gained overall.
* Unlike Apollyon's Void, Abyss cannot be given to other characters who are lacking in damage, as it is in Tainted Apollyon's consumable slot.
* Tainted Apollyon will want {{i|BFFS!}} and {{i|Hive Mind}} more than other characters, due to the fact that Abyss' locusts are boosted by these items.
** This also means he will be competing with {{c|Lilith}} and {{c|Tainted Eve}} for familiar-boosting items.

=== {{C|Tainted Forgotten}} ===

; Strengths
: High damage
: Flight
: Bone pile can stay next to bosses and deal constant damage
: Free Curse Rooms
; Weaknesses
: Can only have soul hearts for health
: Very low range
: Bone pile cannot move on its own; the soul must get back to the bone pile in order to reposition it if it's not close to enemies
: Speed ups are half as effective

* Only the bone pile will receive the effects of items, making Tainted Forgotten value items very differently from other characters. Given the fact that it can't move without The Soul's help, is indestructible, and wants to be close to enemies at all times, this makes Tainted Forgotten want the following items more than other characters:
** Size ups (to block shots and increase the range of the bone club)
** [[Familiar#Orbital Familiars|Orbitals]]
** Contact damage
** Area-of-effect damage
** Creep
*** As these aren't generally useful to other characters, Tainted Forgotten doesn't have to compete nearly as much for the items they want compared to other characters.
* In a similar vein, this also makes Tainted Forgotten want the following items less than other characters:
** Shooting familiars
** Bomb modifiers
** Size downs
** [[Status Effects|Fear/Confusion]]
* The bone pile can be thrown into and out of Curse Room doors or at [[Chests#Spiked Chest|spiked chests and mimics]] without spending health.
* The Forgotten's interactions with tear modifiers should be considered when choosing who to give them to.

Aside from the obvious drawback of being limited to soul hearts for health, Tainted Forgotten's unique interactions with items makes them value items much differently than other characters, and is the main reason to consider them as a co-op partner. Size downs decrease their ability to deal damage, shooting familiars will be very clunky to use, and on-hit effects will apply to the bone pile instead of the soul, making them much less useful for Tainted Forgotten and better given to other characters. However, any close-range damage sources (such as {{i|Demon Baby}} and  {{i|Monstrance}}) and contact damage (like {{i|Midas' Touch}}, {{i|The Virus}}, and {{i|Mom's Heels}}) can be used to excellent effect when Tainted Forgotten's damage source can't take damage and wants to be close to enemies at all times. As these items aren't generally as useful when used by other characters, Tainted Forgotten doesn't have to compete with other characters nearly as much for the items they want to have.

Aside from this unique mechanic, Tainted Forgotten's flight lets them get pickups behind gaps and rocks for free, and they can enter Curse Rooms and open spiked chests for free by throwing the bone pile at them.

=== {{C|Tainted Bethany}} ===

; Strengths
: {{i|Lemegeton}}
: Can use red hearts to use her active item(s) more often
: Allows other players to pick up more items, as Lemegeton can provide her own source of items.
; Weaknesses
: Can only have soul hearts for health
: Stat-ups and familiars are 25% less effective for her

* Lemegeton is in Tainted Bethany's consumable slot, which lets her hold another active item that she can use blood charges on.
** Characters whose active items don't tie directly into their game plan can give their active item to Tainted Bethany, as she'll be able to use them more frequently.
* The value of red hearts varies greatly depending on the other characters in the game and their current health; consider who wants/needs them more at the given time.
** {{c|Samson}}, {{c|Lilith}}, and {{c|Tainted Eve}} will especially be competing with Tainted Bethany for hearts to use.
** If the other characters can't use soul hearts (both forms of {{c|The Lost}}, {{c|Keeper}}, plus {{c|Tainted Magdalene}}), Bethany is free to use red hearts for her active item(s).
* Tainted Bethany will want familiars less than other characters, due to the fact that they inherently have less of an effect when she has them.
* Tainted Bethany with Tainted Magdalene is an interesting combo. because the chance for emeins to drop red hearts on death speands to all characters, this means Tainted Bethany could get more charges then normal. However, because of the fact Tainted Magdalene bleeds her hearts out, red hearts on the floor should not be picked up Tainted Bethany

=== {{C|Tainted Jacob}} ===

; Strengths
: Slightly increased tears
: {{i|Anima Sola}}
: [[Dark Esau]] deals heavy damage to enemies he runs into
: Can turn into ghost form at any time if desired to get out of difficult situations
: While in ghost form:
:: Flight
:: Can take one Devil Room item for free each floor  Useful even when dead, as Dark Esau remains and chases his ghost baby, or will spawn next floor even when he's dead and (unsuccessfully) chase other players
; Weaknesses
: Turns into a version of {{c|The Lost}} ''without'' {{i|Holy Mantle}} for the rest of the floor if Dark Esau touches him

* Dark Esau is unable to hurt other characters
** Wisps spawned by {{c|Bethany}}'s {{i|Book of Virtues}} and {{c|Tainted Bethany}}'s {{i|Lemegeton}} can damage and be destroyed by Dark Esau.
* If Tainted Jacob dies, Dark Esau will continue to attack only him, even though he can't be hurt anymore. This allows Dark Esau to be used purely offensively.
** If Tainted Jacob is dead at the start of the floor, Dark Esau will target other characters instead of Tainted Jacob, but will still be unable to hurt them.
* If Tainted Jacob was in ghost form when he died, he will respawn in ghost form after the boss room is cleared, so players should double check the room and Dark Esau's location before the boss dies.
* Dark Esau can open the doors to [[Boss Rush]] and [[Hush]] if he's kept alive to [[Mom]] and [[It Lives]], respectively.

Tainted Jacob is a very odd Co-Op Character to play as.  Dark Esau will only affect him as long as he is the only Tainted Jacob in the match. After he dies, Dark Esau will continue to hunt down his ghost baby but will never kill him. Since it does not hurt any other players except during his spawning animation, Dark Esau can be used as a way to attack other enemies and render the damage value of 0.50 as a ghost mute, as oftentimes, the ghost can go through bigger bosses, such as Hush. If Tainted Jacob entered his The Lost form prior to dying, he will be revived in that form.

Keep in mind that in his Lost from, Tainted Jacob can take one free devil deal item when in his lost from, however, this puts him at odds with {{C|Bethany}}, who has an increased ability to acquire Angel Rooms because of {{I|Book of Virtues}} as well as {{C|The Lost}} and {{C|Tainted Lost}} who thrive off of the same devil deals.

If Tainted Jacob is dead at the start of a floor, Dark Esau will still spawn, and will try to hunt down another player, but will be unable to kill them if they aren't also Tainted Jacob. In a match with multiple Tainted Jacobs, contact with any of the Dark Esau's will kill a Tainted Jacob, {{I|Anima Sola}} will only work on the Dark Esau that belongs to the Tainted Jacob using it, and the multiple Dark Esau's may choose to pursue the one living Tainted Jacob if the others are completely dead and turned into ghost babies. In general, playing multiple Tainted Jacobs is not a good idea unless every player knows what they're doing.

== Bugs ==
{{Bug|When one player is using Tainted Cain and takes a boss item pedestal, all items that aren't claimed will be taken immediately.}}
{{Bug|If Tainted Jacob is revived in his The Lost form and killed before the next floor starts, his ghost baby will be a floating Isaac head and turn into a floating Isaac torso when moving. This is purely cosmetic.}}
{{Bug|When an active item is on a boss pedestal and a player that already has an active item picks it up, any item underneath will despawn and only the replaced active item will be on the pedestal.}}
{{Bug|If Jacob & Esau is the last person to die in a co-op game, the game will not end and will instead softlock with neither of you able to leave the room. This is also true with Tainted Forgotten.}}
{{Bug|If multiple people are playing as Jacob & Esau, Esau's health will only be shown for the first player to join the game.}}
{{Bug|If you exit the game to the desktop and after returning to the game you click "continue" on a different controller/keyboard than the one you started the run with before,  the second player's character will change into the character of the first player. Health bar of the second player will not be visible unless you exit to the menu and return to the game.}}
{{Bug|On Xbox, when loading into a challenge, if the second player quickly presses left or right on the d-pad while spawning in the second player will be able to either choose random or Maggy, this random character will spawn with the items granted by the challenge  along with their normal starting items, however their normal starting active will be removed. The hud for character selection will briefly appear if done correctly (This exploit is very inconsistent and can seemingly only be done while the first room of the challenge is fading in from the loading screen). }}
{{Bug|If Bethany dies by any source that removes heart containers (Guppy's Paw, Potato Peeler, etc.), she will respawn with one half soul heart (one and a half in beta online).}}

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