It's repentance exclusive. To get them, you need the red key or a cracked key. An easy method of getting a cracked key is to drop a trinket in a boss room. Upon going through your ascension the trinket you dropped in the boss room will turn into a cracked key. Get that cracked key, use it in the hallway leading into Mom's Bedroom and a door to a closet will open. Inside the closet, depending on what character you're playing as, a tainted version of your character will be on the floor in a fetal position, crying. Touching the tainted doppelganger will cause it to stand up and shoot into the heavens, causing you to unlock that tainted character.