-Vanitas is Ventus' darkness. He looks like a black-haired Sora.
-Terra's final battle is Master Xehanort. Xehanort steals his body, and Terra summons his armor as a temporary body to defeat Xehanort. The armor becomes the lingering spirit boss in Final Mix+ and Xehanort in Terra's body becomes Ansem's apprentice.
-Terra is the one responsible for Braig's/ Xigbar's missing eye. He shoots it out in a conflict at Radiant Garden.
-Aqua's final boss is Ventus possessed by Vanitas. In earlier trailers, when Xehanort tells Ventus that "he has lost it" he is referring to his darkness, Vanitas.
-No one has beaten Ventus' story yet, but popular belief is that Vanitas is his final boss. No news on why he looks like Roxas yet.