Characters are cards used by players. They all have 2 hit points, 1 damage, an activated ability to play an extra loot card, and a unique starting item. The only exceptions to this are Eden, who can choose their starting item, and The Lost, who starts with an extra soul, at the cost of having one health point.
Originally, only four characters (Isaac, Magdalene, Cain and Judas) were to be in the base game. Azazel, Apollyon, The Keeper and The Lost were Gold Box exclusive, and the rest were unlocked by stretch goals to be put in all boxes.
List of cards
Base game
KickStarter Expansion Pack
Plus, some tiers of the kickstarter got an unique personalized character-styled card
Four Souls+ Expansion Pack
The official site shows Guppy twice, as well as a duplicated nickel, but they are actually mistakes replacing "Divorce Paper" and "Libra" Treasure cards