Overall, he's the guy you are fighting to stop. Through many iterations of himself, he has continuously attempted to obtain control of darkness and the power to re-shape the worlds, the universe by his own ideology. He is the leader, as his Nobody form Xemnas, of the Organization 13.
There are multiple ways of explaining who Xehanort is so be warned, beyond this point there will be background information containg light story spoilors if you haven't played games beyond the first and second game in the series.
The true Xehanort is a Keyblade Master who once taught Ventus. Master Xehanort believes that darkness and light should exist in perfect balance, and for this reason, he seeks to conquer Kingdom Hearts and recreate the worlds according to his wishes. Through his many different incarnations, including Terra-Xehanort, his Heartless, Ansem, Seeker of Darkness, and his Nobody, Xemnas, Xehanort is ultimately responsible for the series of events that repeatedly threaten the worlds with destruction, including the mass release of Heartless into the Realm of Light and the creation of the Nobodies and Organization XIII. He has also brought tragedy to many that have crossed his path, including Sora, Riku, Kairi, Terra, Ventus and Aqua.
~ NecroGasmic♥ 14:49, December 5, 2013 (UTC)