Tainted Keeper starts with 2 coin hearts and 1 bomb. Tainted Keeper shares the same mechanics with the regular Keeper, including recovering one coin heart upon picking up a coin, as well as converting any heart pickups to blue flies. Picking up a coin to restore a heart does not add a coin to his coin counter. Tainted Keeper shoots four tears at a time, similar to the item Mutant Spider. Whenever an enemy is defeated, a random coin is dropped that will vanish quickly. Most items encountered when playing as Tainted Keeper must first be paid for with coins, including Deals with the Devil, Angel Rooms, Boss Room items, and Treasure Room items.
In the standard game mode, Shops are different and much better for Tainted Keeper. They do not require a key, spawn with two Shopkeepers, and have a vastly increased stock available. At max level, they have eight items/pickups for sale, of which there will always be at least one Shop item, one Boss Room item, and two Treasure Room items. The other four terminals in the shop can be keys, bombs, cards, pills, trinkets (which will be sold for 5 cents), or more Shop/Treasure Room items.
This shop can rarely be replaced by a better (or worse) variant: The better variant has 2-6 more shopkeepers, Urns scattered about, two Lucky Pennies, and one more random item/pickup for sale. The worse variant has only one random sale terminal, one Shopkeeper, some Poops, and a Fly.
Enemies spawned after entering the room (i.e. spawned by another enemy or spawned on later waves in Challenge Rooms) don't have a guaranteed coin drop.
Spawned enemies will stop dropping coins after fights go on long enough such as with Boss Rush.
Champion enemies and Mini-Bosses drop 2 coins on death.
Bosses drop several coins.
If the boss is comprised of multiple enemies (e.g. Gurglings), each of them drops fewer coins.
Most degraded bosses drop 3 coins, exceptions such as The Cage and Monstro II have regular boss coin drops.
Coins dropped by bosses stay on the floor longer before dissappearing.
Boss Rush items must still be paid for, and doing so will start the event.
Because Tainted Keeper has to pay for all items, taking devil deals is often more powerful than taking Angel items. This is because taking one devil deal item will not despawn the other items, but only one item can be taken from an Angel room.
Sticky Nickels are able to spawn as a result of killing enemies, effectively making them impossible to collect (aside from having an accompanying explosion to break it free after spawning, and being mindful enough to collect it in time), as it would have already disappeared by the time a bomb detonated to pick it up. Remote Detonator would help, but it could end up in self-damage as the explosion would have to be extremely quick.
Coin drops from enemies are generated when a room is first generated. If an enemy that begins in the room drops a nickel, dime, or penny variant, it will drop the same coin if you leave and re-enter the room. This makes items such as Mercurius and Ventricle Razor much more useful, as they allow the player to easily leave and re-enter the desired room, allowing the player to easily generate money to fuel the run.
In Co-Op, all players will need to use money to purchase items if Tainted Keeper is in the game, but enemies will also drop money regardless of who killed them.
With this in mind, having the Blessed Penny can earn a huge amount of Soul Hearts for the players who can use them quickly.
This can be used to get Devil Deal items without losing health for all players.
Tainted Keeper can utilize Sacrifice Rooms much more reliably and efficiently than other characters, because monsters reliably generate health. This can be done by leaving regular coin drops on the floor to go pick up, or alternating stepping on the sacrifice spikes with clearing a monster room. The latter is reliable to produce coins but risky because you're always entering an unknown monster room with one health, but can be used with restarts to start a run with one or two angel room items. Keep in mind that Tainted Keeper has to pay for items in Angel Rooms that you may get at the 6th sacrifice, though the item drop you may get at the 7th sacrifice will be free.
Having a good speed stat may be very beneficial, in order to pick up coins before they disappear.
Golden Pennies are able to spawn as a result of killing enemies. It still despawns like a normal coin unless picked up, in which case, any subsequent spawns will not disappear.
Birthright strongly attracts coins dropped by defeating enemies to Tainted Keeper, doesn't affect other coins.
Magneto acts as a slightly less effective version of this.
Blood Rights effectively allows for free damage as long as you can get to a dropped coin.
The Boomerang/Jaw Bone allow for Tainted Keeper to retrieve dropped coins that are otherwise inaccessible, such as ones over a pit or a gap.
Most Penny Trinkets are powerful from the sheer amount of coins Tainted Keeper is able to earn.
Burnt Penny in particular is especially powerful; in a room with enough enemies, it is possible to gain infinite money and bombs by killing most enemies in the room, then bombing your way out of the room to reset it.
D7 Because coin drops are determined by the seed, the D7 can be used to reset a room with a valuable coin to get it twice. Can additionally be used in Shops and Secret Rooms to refight Greed and Super Greed, allowing for large coin generation.
Deep Pockets guarantees 1-3 coins when clearing rooms if they would otherwise have no drops, and increases max number of coins to 999.
Door Stop effectively allows for infinite money with enough patience. Tainted Keeper can leave 1 monster alive in a room, leave, and re-enter to collect even more coins from killing enemies. Since the coin drops are determined by the seed, you can wait until an enemy drops a nickel or dime to do this to spend less time.
Fruit Cake can have a consistently positive effect overall, as you gain the Eye of Greed, and Head of the Keeper chance of effects to your tears, as well as the overall other tear modifiers, on top of beginning with the 4 tears per shot, instead of 3, further increasing the chances of multiple effects, allowing you to have a much higher chance of generating coins.
Greed's Gullet works very well to address Tainted Keeper's low health, able to grant up to 4 additional coin hearts, and up to 12 additional containers if you also have Deep Pockets. Keep in mind that you do have to buy most of your things, and that lowering your coins below each threshold will also remove a coin heart.
Like Keeper, Tainted Keeper can technically have unlimited coin hearts by swallowing many copies of Mother's Kiss.
Humbling Bundle has a chance to double the coins dropped from killing enemies.
Isaac's Heart makes Tainted Keeper's body invulnerable and deals contact damage, makes it much easier to collect coins. It also makes a strong synergy with Midas' Touch.
Keeper's Sack gives you stat upgrades every time you purchase an item, which is available much more often with Tainted Keeper than with other characters.
Magic Fingers can be used to kill every enemy in the room, then you can gather their dropped coins to replace the ones you spent. Since enemies can also drop nickels and dimes, you'll tend to gain money over time. The item is especially useful with high damage, or on later floors where you don't need to save your money to buy items.
Monstrance's damaging aura helps compensate for Tainted Keeper's low tears stat, and can destroy large amounts of enemies at short range so you can focus on collecting the money.
Proptosis's downsides aren't as severe, as you are usually already close to enemies to collect dropped coins.
Silver Dollar grants you access to two extra shops after Mom, which is extra valuable with Tainted Keeper's stronger shops.
Steam Sale reduces prices of all items, even outside the shop.
It is almost always worth it to buy items that gives Bombs and Keys on Pickup, as most are +5. A single bomb or key usually goes for 5 pennies, and an item is 15 pennies. You save 10 pennies buying a item that gives pickups instead of buying them outright.
Virgo and Celtic Cross has a chance of allowing Tainted Keeper to safely regain lost health and collect coins after getting hit; this is very good for killing large groups of enemies.
Fanny Pack can be used to break the game in a room with many enemies by taking damage and killing enemies to heal back the lost health. Fanny Pack may provide bombs when you take damage, allowing you to continuously bomb out of the room and re-enter.
Mother's Kiss will allow Tainted Keeper to have 3 Coin hearts, which is the only method other than Greed's Gullet to harbor more than 2 Coin hearts. If the trinket is dropped, the extra Coin heart will disappear. Gulping it will add a permanent Coin Heart.
Sigil of Baphomet can be used to let Tainted Keeper grab coins without risk of taking contact damage after killing an enemy.
HP Up items (Apart from Greed's Gullet and Mother's Kiss) don't do anything. Unlike regular Keeper, Tainted Keeper can never have 3 hearts. Only take them if your health has been reduced in any way (such as by taking Dead Cat).
While Bum Friend or Bumbo would be items normal Keeper would want to stay away from, Tainted Keeper could potentially benefit from these items considering the large amount of coins he is able to produce.
Butt Penny is useful for pushing away and poisoning enemies, but can cause other coins to be pushed away and time out.
Experimental Treatment has a chance to grant or remove 1 coin heart, making it a more favorable item if an HP up is nearby but can be detrimental if there isn't.
Pageant Boy is generally a bad item, as it usually pays out with 7 cents and costs 15. However, if it is on sale, it is almost always worth picking up, as it will always at least pay for itself, and has the opportunity to net a profit if a more valuable coin spawns such as a Nickel or Dime.
20/20: Decreases damage to 75%, but increases the default tear stat, making it 2.73 without other items and effects. This allows him to double his damage output with tears. Decreases the spread of tears fired, making it easier to hit a single enemy with more tears.
As Tainted Keeper can reach 5.0 tear rate cap, this item becomes less effective damage-wise by the time Tainted Keeper gets to 3.75 tear rate without 20/20 and becomes disfavored after that, overally making him output less damage per second with tears.
Currently Damocles does not work with Tainted Keeper, as every item needs to be purchased.
Items that decrease Tainted Keeper's size like Mini Mush and Pluto should be avoided, as the smaller hitbox makes it harder to collect coins dropped by enemies.
Items that deal damage to every enemy in a room, such as Toxic Shock and Butt Bombs, can make it difficult for Tainted Keeper to collect coins as they will spawn rapidly across the entire room as multiple enemies die at once.
Heartbreak: Immediately turns the second coin heart into a broken coin heart, and the next hit will kill Tainted Keeper.
Mutant Spider: Tainted Keeper will now shoot six tears with x0.42 tears modifier. This makes his damage output per second with tears smaller by ~36%.
The Inner Eye: Tainted Keeper will now shoot five tears with x0.51 tears modifier. This makes his damage output per second with tears smaller by ~38%.
Meat Cleaver: Multiplied enemies will not drop coins: Only an amount of coins equal to the original amount of enemies in the room at the start can be dropped.
Sack Head: The pennies dropped from killing enemies have a chance to be sacks instead. The sacks do not flash and disappear.
Similarly to how the chance for dimes and nickels to appear are determined by the seed, sacks are as well: rerolling the enemies in a room with D7 will result in them dropping sacks again if they had the first time, with the same exact contents inside the sack every time.
Hematemesis pill makes Tainted Keeper spawn coins instead.