Isaac (アイザック Aizakku) is the titular character and the first available character in both games.
Isaac's stats are simple, and he is considered the base of all characters.
Isaac starts with no activated item, but after beating The Womb with ???, Isaac gains The Dice. In Rebirth and Afterbirth, ??? must defeat Isaac for Isaac to start with The D6.
In the Cathedral, Isaac also appears as the main boss.