Unfortunately, you won't continue to find the Red Key in Mom's Room in later runs. Touch that character to unlock them for future runs. It will open a hidden room with the tainted version of the character you are playing as inside.

Go into the hall outside of Mom's Room and use the Red Key. You can now use this item to unlock Tainted Characters. Open the chest here to find the Red Key item, unlocking it for future runs. It isn't randomized like the other areas of The Binding of Isaac, so work your way through the few rooms and you'll get to Mom's Room. Go all the way through the Ascension and you'll reach a new area called Home. You'll find Dad's Note inside this room and you'll begin your ascension. Return to the starting room after defeating the boss, and you'll find a door that can be opened with The Polaroid, The Negative or the Faded Polaroid. Look for a slightly different skull somewhere in the level and bomb it in order to find a Fool card if you don't have teleportation.

To find it, you'll need to defeat the boss and then teleport out of the room. This is a special item you'll be able to grab in the biome Depths II. You'll need to find Dad's Note on another run. This is a new boss added to the Repentance DLC, and you'll find her at the end of the Corpse II biome. You'll need to defeat Mother at least once. However, it is time consuming if you want to catch 'em all, and you need to string together a good run in order to reach the point where you can access them. Unlocking tainted characters is simple to explain, as the process is the same for each one. Here's how to unlock each tainted character and what makes them special. Even though they look like reskins of Cain, Isaac, Lilith, and the rest, most of them start with new items and abilities, effectively doubling the way you can begin a new run once they're unlocked.

Every playable character in The Binding of Isaac: Repentance has a 'tainted' version.

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