I don't think that DiZ made xion, because he didn't have the power to do it, he just use the "existing" nobodies and stuffs for awake sora
I think this...when kairi first lose his heart and gave it to sora, xion was created, later when sora lost his heart and kairi's heart in the sacrifice of KH I , roxas was created of the separation of sora's heart and body, and namine was created of the separation of kairi's heart of sora's body...its like roxas is just "son" of sora, xion just "son" of kairi, and namine's the "son" of both of them, for that she has powers over sora's memories ... and xion cant endure it, because always she has only sora to want beside her, but now she also has roxas, but chosee one of them is kill the other, or maybe the creation of a second nobody from the same heart began the destruction of the first, and we need also put the behind history of ventus(that is obvius that something happens in the past and for something ventus = roxas) that anyone know and anyone can answer this before play the game xDDD