Who is nobody is xion

Xion is the nobody of what fragmented memory Ventus (Roxas in this part of the series) has of Aqua. YES I know she is also an experiment Vexen created but if you actually play Chain of Memories and 358 you find out she has Sora's memories and also takes Roxas' being away slowly. Now why did I bring Birth By Sleep into this? Well play BBS pay attention to all details and watch that in the special ending video Xemnas sits in the room of awakening and talks to aquas armor, the fact Ventus isn't there asleep means during KH 1 when Sora's used Riku's Keyblade to unlock his heart and awaken kairi he also sent Ventus' heart back to the body left behind in the room of awakening. Now if you beat BBS you notice when Xehanort tries to separate Terra from his own body that afterwards he loses all memory of being Terra. Same went to Ventus. He lost all memory of his past and only fragments Sora had got through to Xion, hence why she has short black hair and a face resembling Aqua's.