The are multiple characters that can be unlocked in the game, each requiring you to fulfill a certain condition during a play-through. Note: You can only unlock characters in regular, random runs. Meeting the requirements in a challenge or manually inputting a random seed will not allow you to unlock characters (indicated by the no trophy icon below your stats).
If you type in one of these codes as a seed, you can play a special run with unique features. Unlike normal seeds, the floor layout will be different each time. However, achievements & unlocks will be temporarily disabled.