Eh, could we please stop with this nonsense. Only Sora is Sora (and Roxas is his Nobody). That's hardly a superhard concept to grasp. The rest are completely separate good guys.Eh, if you know that Sora is all the good guys, you already know the converse for the bad guys.
Though, you are right with the bad guy thing, but that makes perfect sense in an "evil guy uses darkness to extend his life" way. I like it that Xehanort is the one big bad in the game and all of his "forms" make sense within the KH Universe's rules (embracing the darkness, body possession, Heartless & Nobody forms).