Question: how to unlock binding of isaac afterbirth characters?
Answer: Isaac: Unlocked by default. ... 
Magdalene: Have 7 heart containers at the same time. ... 
Cain: Have 55 coins at any given time. ... 
Judas: Defeat Mom's Heart.
Eve: Make at least two deals with the devil in one run.
???: Defeat It Lives (a more difficult version of Mum's Heart, unlocked after beating Mum's Heart 9 times).