Spoiler It's because she's a copy of him ... which happened to show up in the form of a girl. Yeah~
Sorry to burst anyone's romantic bubble but ... Spoiler Xion is really just a copy of Sora that was transformed into a girl body. It'd be just like the pairings being Riku and Sora ... except for the fact that Sora is gender switched. Yeah ... those yaoi fangirl's dreams have come true.
??? Huh, what?
