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 . . .