Well it is possible they just through it in "for shits n giggles"....actually, I like that theory. But maybe MX is already a heartless, and kept human form like AnsemSoD....I mean, I think MX mentioned giving in to the darkness....
And I thought the battle actually took place after the final battle, because isn't he like the extra secret final boss?
Anyway, your all probably right that it is a new character with a connection to MX.