Original: The Ansem whom Sora defeated was actually the Heartless of Xehanort , a student of Ansem the Wise , and that the leader of the Organization is Xehanort 's Nobody , Xemnas .
Simplified: Xemnas is the Nobody of a man called Xehanort , who no longer exists . Ansem , the main enemy from the first game , was Xehanort 's Heartless .