Blue Baby's Soul (actually known as ???'s Soul) is a trinket added to The Binding of Isaac: Rebirth. It appear as a little ghost similar to ???.
It is unlocked after beating The Dark Room level with ???.
- Spawns the nearly visible Brother Bobby-like familiar that floats slowly around the room on a zig-zag pattern similar to The Peeper.
- Fires spectral and homing tears.