Ok i know this may seem like a stupid question but I need an explanation to the whole Ansem, xehanort, xemnas thing.
I understand. There's at least 4 characters in the series who at one point either hold the name Ansem or are thought to be Ansem or can to some extent be referred to as Ansem without the person being totally far off. Here's the deal:
DiZ is the real Ansem. He is not evil, but he made some mistakes. That's one of the biggest shockers of the game. In no point or time in the Kingdom Hearts series is Ansem a bad guy. He was a stickler for revenge during Roxas's story, but he reformed.
The Ansem we beat in the first game was actually his apprentice, Xehanort. Xehanort was just going around calling himself Ansem. Why? Because he's a big meanie head. Besides, just because he wanted to be the guy who causes mass genocide on a universal scale doesn't mean he wants to be KNOWN as the guy who does it.
Xenmas is Xehanort's Nobody, not Ansem's. At no point does Ansem turn into a Heartles (he dies when his Kingdom Hearts Encoder explodes, if I'm not mistaken, so he's just dead). I don't know why Xenmas took a name that's an anagram of a totally different person. Maybe since Xehanort wanted to be known as Ansem and Xenmas is just half of Xehanort, they kind of have the same desires in that sense. Or maybe Xehanort's name is already hard enough to pronounce without jumbling the letters around and adding MORE freakin' X's.
Riku goes by the name Ansem twice, though who he thinks is Ansem is really Xehanort. In the first game, Xehanort possesses his body. In COM, it is revealed that part of Xehanort still rests inside his heart. In Kingdom Hearts 2, he had allowed darkness to overtake him just enough so he could wield its power, but not enough to control him like in KH1. Him taking the form of Xehanort was just one of those mild side effects, like the ones you get from prescription drugs (Riku probably also got headaches, nausea, diarrhea, coughinh, sore throat, and anal leakage). So the guy in the beginning with DiZ (the REAL, non-evil, Ansem the Wise) calling himself Ansem was Riku taking the form of Xehanort (I had thought that it was Xenmas in the beginning. Oopsie).
Hope that clears things up.