Could it be possible that maybe Xion is the remmants of Sora's memories of Kairi, when Namine went and manipulated them? Because Namine did say that there are memories deep down in one's heart that she cannot reach. Even though Sora may have forgotten Kairi's name, there might have been some memories of her deep in his heart and those memories came together to create Xion. Because Xion was only a combination of only parts of Sora's memories of Kairi, the more Namine restored the memories of the real Kairi, the more Xion disappeared. It would kind of explain why Xion looks like Kairi and could also be a nobody, because nobodies are made out of darkness and memories of a someone, Xion was created inside Sora and who had become a heartless before, therefore proving he had darkness inside of him. I know, its a bit of a stretch, but it could happen...