OMG WE HAVE A WINNER WITH THE COMPLETELY ORIGINAL AND NEVER BEFORE HEARD OF THEORY.this was probably mentioned but i'm giong to makea thread about it anyways....
ok, in BBS we know that master Eraqus and terra's heart fused....so in the last episode MX mentions that terra has succumbed to darkness. So let's just say this counts as becoming a heartless. In order to gain a nobody, you must gain a heartless (most cases).
So here's my theory: this is master eraqus's and terra's nobody (in the same way roxas is both sora's and ventus's). this could explain the similar fighting style and similar weapons to Xemnas.
This could also explain how Xemnas thought of the idea of using the cloaks as a regular suit (and then a uniform later)