My first idea as to who Xion is would be that she could be the Nobody of both Sora and Kairi since their hearts were once one, however brief. Mostly since, in my opinion, looks more like Kairi than Namine does. Also she is able to use the Keyblade much like Sora. This would kind of be the case since it was shown before Kairi was shown using a keyblade.
My second guess would be Xion could be the unbirth of Sora and Kairi, though so far unlikely. Since it is still yeat unrevealed as to how they are created or even how they may have become exstinct, if at all. This guess would mostly lean towards the facts that we stilll know very little about the Unbirths, and will be a while before we do.
Out of the two theories, I mostly support the first one than the second.