Oh, I forgot about this KH-meets-Nanoha theory...
Xemnas is a Dragon Summoner
Remember the final battle(s) of KH2? How Xemnas has that giant mecha-dragon thing? Well, Xemnas having something of a summon ability, similar to Caro's, helps that entire sequence suddenly makes sense.
...okay, how do I elaborate on this without giving out MASSIVE spoilers?
...So, imagine his Somebody, before ANY other point in the story of KH, came from Alzas and had a hidden talent for dragon summoning. When he became a Nobody, this power was amplified tenfold, though he only has one summon: the Mecha-dragon of doom.
Also, Xemnas is distantly related to Caro. Look at 'em;
Here is Xemnas:
(I seriously want to run my fingers through his hair. I would die but it'd be totally worth it)
And, of course, Caro:
...well, I see a resemblance.
Also...