Characters

There are 34 playable characters in The Binding of Isaac: Rebirth, with 11 available in the base game and 23 available in the DLCs. Five of these characters (,, , , and ) came with the original The Binding of Isaac, while two additional characters ( and ) were added later. Eve was added in The Binding of Isaac 's Halloween Update, while Samson was added in the Wrath of the Lamb expansion. Rebirth contains four new characters (,, , and ). In the Afterbirth expansion, one new character was added, plus one more character via an update. In the Afterbirth † expansion, one character was added, plus one more secret character  added via a Booster Pack. In the Repentance expansion, two characters ( and ) were added. Each character starts with different stats and abilities that give them their own unique playstyle.

Repentance also added an alternate 'tainted' version of all 17 characters, unlockable in the expansion's final endgame area. Each tainted character maintains the same general theme as the original incarnation, but twisted or taken to an extreme. Many have unique, game-changing mechanics, but often come with a corresponding drawback. Tainted characters have their own set of completion marks and corresponding unlocks, albeit fewer than the original characters — a single unlock for both of the timed challenges (Boss Rush and Hush), one for all of the main path bosses (Isaac, ???, Satan, and The Lamb), and one for each endgame boss, excluding 'normal' Ultra Greed.

Each character's progress can be gauged by checking their completion marks.

Unlocking
Each character is unlocked by accomplishing a certain task in the game. The unlock requirements are as follows:

Trivia

 * One common theory (which was confirmed by Edmund) is that all characters are different forms of Isaac himself. This theory suggests that every character is a creation of Isaac's imagination which he assumes the personality and appearance of when playing as them. Most characters represent different issues that Isaac believes he has, while ??? is Isaac after dying by suffocation inside The Chest, Azazel is who Isaac sees himself as, and The Lost is Isaac's wandering soul (which can be seen upon dying).
 * Additionally, Keeper is Isaac's desiccated corpse, as evidenced by Ending 18.
 * The Forgotten could be Isaac's skeleton shown in Ending 17 and Ending 20.
 * This theory is also confirmed by the cutscene before fighting The Beast, in which Isaac briefly turns into The Lost, The Forgotten, and ??? as he grows his angel wings.
 * While on the character selection screen for the tainted characters, a male voice sings instead of the normal female voice. The same is true when loading into the game after selecting a tainted character.
 * Also, when a tainted character is selected or being selected, the taskbar icon of the game changes to look like Tainted Isaac.


 * In the upcoming Requiem expansion for the Four Souls card game, the tainted characters will have their achievement names (e.g. "The Broken" instead of "Tainted Isaac") unlike in the video game. This was decided in a poll made by Edmund via twitter.
 * Before that, Edmund had posted an image of Isaac's alternate version being referred to as "Tainted Isaac".