This is NOT a fusion character, thank you. Please read more for details.
What do you know, I live.
This is mostly a theory piece. Likely wrong, and it's a shame that the game was released JUST NOW in Japan, rather than me getting this up a few days ago. Oh well.
The idea is that Xion is a shapeshifter.
1) A Japanese translation of a scene had Xion asking Roxas at the clocktower "How do I appear?" When she removed her hood, her face wasn't shown and he looked surprised.
2) She already looks like Kairi in the face, exactly, though her eyes are as bright blue as Sora's. This is a closer similarity than either Roxas or Namine.
3) Timeline: Roxas was "born" when Sora plunged the Dark Keyblade into his own heart (or "attacked himself" as the game put it.) If from then to the time Sora defeated Xehanort's Heartless was a week, that would be surprisingly long. CoM/Re:CoM picks up immediately after KH1. If Sora spent a week in Castle Oblivion before his sleep, I'd again be surprised. So we're looking at less that two weeks that Roxas might have interacted with the Members that were defeated in Castle Oblivion, and likely less. If you don't count when those Members would be at Castle Oblivion, one week. Why would Square put that much effort into such a short timespan, when we theoretically have 358 days to play with? Unless something made them still playable.
4) I understand I've seen more than one "dead" Member playable in Multiplayer. I'm not so sure that holds true for in-game events. That's still something that either I've missed or remains to be seen. Except in Japan.
5) Riku in the ads claimed her Keyblade was fake. But if her power comes from imitation, this makes sense.
6) If she is still alive, it would certainly allow her to have hidden all this time. (However, I speculate she's no longer tangible, aka merged, dead, etc.)
So there's my theory. I may sometime change this picture to just be Xion, pure and simple. Oh well.
Made in Photoshop Elements 4.0 on a Waccom tablet. No vector involved, and completely digital.
Xion, Organization XIII, Kingdom Hearts, Roxas, Vexen, Larxene, Lexeaus, Marluxia, Nobodies, and various other things I have probably forgotten are copyright Disney and Square Enix.