???, commonly known as Blue Baby, is a playable character in The Binding of Isaac: Rebirth. He is a dead baby that has suffocated, hence his blue color. Unlike most playable characters, ??? does not start with any Red Heart Containers, but rather with three Soul Hearts; unlike any other character in the game, ??? can never have red health. When ??? picks up an item that would give him a Red Heart Container (filled or unfilled), it is automatically converted to a Soul Heart. ???'s starting item is The Poop, one of the subjective worst items in the game, unless used creatively as a defense mechanism or in conjunction with other items that enchance it, such as the Petrified Poop trinket.
Trivia
The name Blue Baby comes from Edmund's username on NewGrounds, BlueBaby.
Blue Baby was originally in a game called Dead Baby Dressup!, a game created by Edmund McMillen in 2001. The character is also in the following games:
