DiZ is Ansem the Wise. The TRUE Ansem. The Ansem from the first game was just Xehanort taking on the name of Ansem. But!- it wasn't just Xehanort, it was his Heartless who went by the name: Ansem, Seeker of Darkness. Xehanort in his orginal form, carried on the research Ansem the Wise thought was a mistake, and so took on the name Ansem. When he freely let himself go to the darkness and became captive of it, Ansem, Seeker of Darkness was made as a heartless and Xemnas was created as a nobody. A heartless is composed of a devoured heart (by darkness) and a nobody is composed of an empty vessel (when the heart leaves the body a nobody is formed). So that means everyone nobody, with few exceptions, used to be Human, and now has a counterpart Heartless as well
EDIT: if your still confused just read the Ansem Reports.. they should help