Tainted Characters

Tainted characters are different versions of already existing characters, both in terms of gameplay and design. They are considered completely new characters, raising the total number of characters to 34 in The Binding of Isaac: Repentance.

They are unlocked by using the Red Key, Cracked Key, or Soul of Cain on the left side of the wall in the hall before Mom's Bedroom in Home with each non-Tainted Character.

They can be accessed via the Character select menu by pressing E on the keyboard or RB on controller. The menu will appear a lot more worn with tears in the wallpaper.

As the Tainted Characters, certain unlocks require multiple completion marks: Satan, Isaac, The Lamb, and ??? all contribute to just one unlock with Boss Rush and Hush accounting for one unlock as well. On top of this, Tainted Characters lack an unlock for defeating Mom's Heart / It Lives! on Hard Mode, Ultra Greed, completing all Hard mode completion marks, and naturally lack a corresponding Tainted Character unlock. This results in each only having 7 unlocks rather than the standard 15.

Characters

Notes

The Red Key is guaranteed the first time Isaac enters Home and is unlocked by opening the chest in Mom's bedroom for the first time. After that, Mom's Chest will pull from its own item pool which includes Mom-related items as well as the Red Key rarely.
In order to guarantee an unlock on subsequent trips to Home, Isaac must leave a trinket in either a Treasure Room or a Boss Room. When he reaches the Ascent after picking up Dad's Note, the trinket that was left will transform into a Cracked Key in the corresponding room.
To unlock these characters, Isaac does not need to defeat Dogma or The Beast on the run he opens the closet. They will be unlocked as soon as he touches them (or lets a Bomb explode next to them) regardless of what happens during the following battles.
Tainted characters that start with an active item (except for Tainted Eden) will have that item in their consumable slot rather than the usual active item slot. The item will recharge as expected and can be used normally with the consumable button. Consumables such as cards and pills can still be picked up, and can be swapped with the starting active by pressing the swap button. Other active items will go in the normal active slot.

Trivia

When switching from the normal characters menu to the Tainted characters menu, the music that plays in the background changes to a male singer with a darker tone. The taskbar icon also changes to Tainted Isaac's sprite instead of regular Isaac.
Edmund McMillen has stated in a livestream that the Tainted Characters are the "abused" versions of their counterparts.
Edmund McMillen noted that the Tainted Characters, along with Dogma and The Beast sequence, were initially reserved for a possible Isaac sequel.
There is a consistent theme to nearly every unlock from the Tainted characters:
- a trinket for Satan, Isaac, The Lamb, and ???;
- an environmental object (a Pickup, Machine/Beggar, or a Rock) for Mega Satan;
- a Soul Stone for Boss Rush and Hush;
- a Reversed Tarot Card for Ultra Greedier;
- the starting item for the character, or item themed around the character's unique ability for Delirium;
- another trinket for Mother;
- finally, an item for The Beast.