I looked through the pages, and didn't see this particular theory stated, so here goes.
I think Xion is a /second/ Nobody of Kairi's.
Bear with me, I've got an explanation.
First, Kairi loses her heart to the Heartless the normal way. Sure, there's the /image/ of her body staying there, but couldn't she have had a Nobody created then?
Then of course, Namine is created, but she is created when Sora unlocks his heart and Kairi's with Riku's keyblade.
Thus, since Kairi is a princess, she could be a special case and have TWO Nobodies. Therefore, Namine and Xion.
This would explain why Namine doesn't have an X in her name... because Organization XIII already had a Nobody from Kairi. Xion was just kept secret, just in case Roxas didn't do the job. Would also explain why Roxas seems to be her close friend. The two of them and Axel seem to be a mirror of Sora, Riku, and Kairi, at any rate, though I doubt Axel is Riku's Nobody. Then again, who knows? Riku did lose himself to darkness.
I think Xion is a /second/ Nobody of Kairi's.
Bear with me, I've got an explanation.
First, Kairi loses her heart to the Heartless the normal way. Sure, there's the /image/ of her body staying there, but couldn't she have had a Nobody created then?
Then of course, Namine is created, but she is created when Sora unlocks his heart and Kairi's with Riku's keyblade.
Thus, since Kairi is a princess, she could be a special case and have TWO Nobodies. Therefore, Namine and Xion.
This would explain why Namine doesn't have an X in her name... because Organization XIII already had a Nobody from Kairi. Xion was just kept secret, just in case Roxas didn't do the job. Would also explain why Roxas seems to be her close friend. The two of them and Axel seem to be a mirror of Sora, Riku, and Kairi, at any rate, though I doubt Axel is Riku's Nobody. Then again, who knows? Riku did lose himself to darkness.