Xehanort is a former apprentice of Ansem the Wise, the sage-king of Radiant Garden. Having a great curiosity about the true nature of the heart, especially the darkness that lingers within it, Xehanort, along with five other apprentices of Ansem, conducted horrible experiments on abducted people, resulting in the creation of the creatures now known as the "Heartless".
Xehanort eventually took his master's name and wrote a detailed report on his findings. When Ansem the Wise discovers these inhumane experiments, Xehanort banishes him to the Realm of Nothingness with his newfound dark powers and used the Heartless to ravage other worlds.
His ultimate plan is to collect enough hearts to form Kingdom Hearts, the heart in which all that exist are evolved from.
With Kingdom Hearts, he would obtain ultimate power, control and knowledge.
Xehanort gave in to the darkness and split into two beings: "Ansem", the main antagonist of the first game, and Xemnas, the leader of Organization XIII and the main antagonist of the second game.
Xehanort, as "Ansem", is voiced by Billy Zane in the English version of the first Kingdom Hearts game. In all other appearances, he is voiced by Richard Epcar.