In the 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.
Changes
Certain changes are made to the game whenever multiple player characters are present. They are:
- When a boss is defeated, only one item pedestal will spawn. Whenever a player takes an item from it, another will spawn until all players have taken an item.
- This applies to any item pedestal spawned in the boss room and the second item will always come from the Boss (Item Pool).
- Players who die will turn into small ghost babies (as if taking the item Pluto) upon death. These babies cannot fly, aggro enemies, or pick up items. They can still flip switches on the ground. They have a damage value of 0.50, though some babies can acquire more. When the floor boss is defeated, all dead players will respawn with half a heart depending on what character they are playing.
General Strategy
- Because all coins, bombs, and keys are shared it usually doesn't matter which player takes them. Take note if one player has a trinket such as
Bloody Penny, since the trinket effect will only trigger for the player who takes the coin. Certain trinkets will activate their effect for all players.
- Players must carefully consider what characters they use together. Some combinations will have inherent advantages, such as one player using
Isaac and one player using 
??? meaning neither player will be competing for health drops. Other combinations could create more difficulties with each other like multiple players using ??? and being forced to compete for a rarer resource.
- Because there is no way of knowing what item will pop up after the current one on the Boss Pedestal, take time to consider who needs the item most.
- Only one item will spawn in any Treasure Room, meaning players will have to choose who gets any given item.
- 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.
- Bombs or explosive items like
Ipecac or 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 Beast, Dead Cat should be avoided. This is because it will boot all players out of the room if the player holding it dies and resets the life of said player to one after revival. For Boss fights, espcally ones where your more likely to die, this resets the boss's health.
- The Beast is unique, as revial items will not boot you out of the boss room during that fight.
Character Interactions
- Azazel will keep his unique Brimstone as a ghost baby, though it will have the damage penalty.
- Lilith will be able to use her Incubus for the current room after dying, but for the rest of the floor her baby will be unable to shoot any tears.
- Keeper will cause all Devil Deals to be paid for in pennies. This can be beneficial to some characters, but characters like Eve who may want to get rid of heart containers may suffer. He will also maintain his inherent triple shot as a ghost baby.
- Tainted Keeper will cause all items to require pennies before being picked up, incurring the same upsides and downsides as Keeper's Devil Deal effect. Any player who kills an enemy will cause them to drop a coin that vanishes after a few seconds. He will maintain his quadruple shot as a ghost baby.
- Tainted ??? will add his unique poop counter to the normal bomb counter as long as at least one player isn't playing as him. Regular bombs will be added back to the pickup pool, and any player may pick up poop drops to refill Tainted ???'s supply.
- Tainted Jacob's 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. Dark Esau's spawning animation will deal damage to all players regardless. If Tainted Jacob entered his The Lost form prior to dying, he will be revived at the boss that way. 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 Esaus will kill a Tainted Jacob, Anima Sola will only work on the Dark Esau that belongs to the Tainted Jacob using it, and the multiple Dark Esaus may choose to pursue the one living Tainted Jacob if the others are completely dead and turned into ghost babies.
- 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 gain the Sumptorium as usual, allowing for a higher overall damage output.
- Tainted Azazel can still make use of his unique brimstone and sneeze abilities, but the damage penalty will still apply. The sneeze will also be smaller.
- Jacob and Esau will have 2 ghost babies after death, Esau's ghost baby arriving shortly after Jacob's
Isaac
Isaac generally works well with any character, due to his neutral stats and variable game plan. If 
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 
Eternal D6 to remove the danger of deleting an item altogether.
Magdalene
Magdalene can work as both a "healer" and a "tank" character. Her high vitality means that she can block bullets or enemies from other players to protect them, while her 
Yum Heart starting item will heal all other players for half a red heart. This pairs well with characters such as 
Samson or his tainted counterpart, or 
Tainted Eve, due to all of them having a need for disposable red hearts. All the same, it can be useless on characters such as ??? or outright backfire if another player is using 
Eve or possesses 
Dark Prince's Crown, meaning players who want to play Maggie in these situations may need a new active item.
Maggie suffers from a lack of speed, so items such as 
Caffeine Pill could be seen as a priority for her when deciding what players get what items.
Cain
Cain has a generous array of benefits by default. His 
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 
Lucky Foot also means he can take pills spawned by other players, or even the pill that 
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 
Technology 2 should always be given to others, while reserving range increases like 
Mom's Heels for him will show massive benefits.
Judas
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 
Azazel or ???.
Eve
Eve can be somewhat of a high maintenance character to play with. Players that prioritize staying in her 
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.
Samson
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 
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.
Azazel
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.
Lazarus
Lazarus suffers mainly from his below average range and his bad luck stat. Other than that, he's a somewhat neutral character. If his 
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.
Eden
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.
The Lost
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 
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 
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; opon unpausing all players will begin to float upwards at the exact same time.
Lilith
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 potential with 
Box of Friends she can be very strong when kept alive. Lilith pairs well with Magdalene thanks to Yum Heart allowing her to take more damage to activate 
Cambion Conception more often, as well as any character that can protect her in general. Because of her base health, players should avoid pairing her with ??? or 
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.
Keeper
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 
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.
Apollyon
Apollyon is a relatively neutral partner for Co-Op, due to 
Void being usable in the same situations as which it normally would be. It is currently untested if using Void will use all items from a boss pedestal. 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 Isaac, Apollyon/ 
Tainted Apollyon, or 
Tainted Cain, as their special abilities can make Apollyon's Void irrelevant or risky.
Apollyon's synergizes particularly well with 
Tainted Isaac as Apollyon can absorb disposed items and gain stat boosts instead of wastefully leaving them on the pedestal.
The Forgotten
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 Judas may be better with them to avoid taking damage.
Bethany
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 adds 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 
Book of Virtues. Still, when allowed to collect these hearts and given some extra protection from another, Bethany's explosive potential is easily exploited.
Jacob & Esau
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 generally unfavorable to use.
Tainted Isaac
Tainted Isaac is a high potential co-op partner, as his more options benefit works for all players. This means that when paired with Isaac you can cycle through 4 different choices with a single 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.
Bugs
 Bug!
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When one player is using Tainted Cain and takes a boss item pedestal, all items that aren't claimed will be taken immediately. |
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| Bug!
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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. |
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| Bug!
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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. |
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| Bug!
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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. |
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| Bug!
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if multiple people are playing as Jacob & Esau, Esau's health will only be shown for the first player to join the game. |
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