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I'm kinda leaning towards Xion could be Kairi's real Nobody. I don't really know how to back that up, but maybe Namine isn't really Kairi's Nobody..
There have been theories about Namine being an Unbirth though. o3o I'm sure Nomura is doing something unexpected. Or maybe not.
Kairi doesnt really have Namine b/c shes connected to Sora... i think Namine came out as a nobody when Kairis heart left Sora's body in kh1
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...
Well, that's really only in his final form, after he absorbs the remnants of Kingdom Hearts. His regular fighting style/ablilities is borrowed from the Jedi.