The incarnation of darkness chipped away from Ventus:
KH Appearances: 2FM, BbS
※2FM was in the secret movie only
Keyblade: (same type as the Void Gear)
A young boy who wears a mask and whose whole existence is shrouded in darkness. His true identity is the embodiment of darkness darkness which Master Xehanort separated from Ventus’ heart, and he played along with Xehanort’s scheme to fight Ventus and was successful in generating the χ-Blade. However, in the end he was defeated by Ventus and the χ-Blade was lost.
“Follow Terra and see for yourself, how he will become what is no longer himself.”
[BbS]The Unversed are creatures created from Vanitas’ emotions and Vanitas was releasing them in every realm.
Without the mask [BbS]: As a result of Sora making up for what Ventus’ heart lacked, the part which was missing: Vanitas was also affected by this and thus came to have the same face as Sora.