MAJORRRRRRRRRR SPOILERS!!!!!!!!!!!!!

Some enlightenment:

The SHU is actually Xehanort, who was mislabeled Ansem for obvious reasons. The Ansem we knew in KHI was actually a confused Xehanort. You see, Xehanort was convinced he was the real Ansem, though the real Ansem is actually an aged king who later changed his name to DiZ. Now, apparently, Xehanort had a shell called Xemnas, who is NO. 0 in the Organisation XIII . . . the master of all Heartless, though, I admit, I understand this part a lot less than others . . .

I never really get all this this DiZ-XIII Order stuff. And I have beaten KH1 repeatedly, and both stories of COM twice.

We'll figure it all out when we play, hopefully

EDIT: here read this stuff if you dare http://en.wikipedia.org/wiki/XIII_Order

EDIT2: I forgot to say that there is lots of Spoilers in there

Wiki is so useful. I have articles on it.