The heart that fuses with Ventus in the beginning of Birth by sleep.
How do I know that you ask.
The way Vanitas looks like and it (the brand new heart) said that it fuses with Ventus broken one until he is strong enough to win back the part that already did.
Q: Did what?
A: Fuse with another heart.
So it (the brand new heart) knows that Vanitas heart did fuse with another one already (Xehanorts I'm sure). That leads me to the assumption that the brand new heart has knowledge of the future (also they, Ventus and the brand new heart, said to open the door. A door that, at that time, Ventus has no knowledge of yet).
But here is the real proof: at 19:38 Ventus heart says it right then and there.
Which would mean that somewhere between Kingdom Hearts 3D and Kingdom Hearts 3 Sora will cast asside his body or is forced to do so.
Please tell me if I'm wrong or if u think different or what possibilities result from that. Discuss this please, I wanna know!
Edited by RayIII, 04 April 2012 - 06:40 PM.