Tainted Characters

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

They are unlocked by using the Red Key or the Cracked Key on the left side of the wall in the hall before Mom's Bedroom in the Home floor with specific characters.

They can be accessed via the character select menu by pressing E. The menu will appear a lot more worn, with tears in the wallpaper.

As the Tainted characters, certain unlocks require multiple completion marks, this includes Isaac, ???, Satan and The Lamb all contributing to just one unlock, as well as 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, 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.

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.
 * When switching from the normal characters menu to the Tainted characters menu, the window icon is changed from normal Isaac to his counterpart.
 * The same happens behind the character select paper when switching.
 * There is a consistent theme to nearly every unlock from the Tainted characters, a trinket for Isaac, ???, Satan, and The Lamb, 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, and finally an item for The Beast.