Xemnas, also known as the "Superior," is a Nobody, and the leader of Organization XIII. He was born when Xehanort turned himself into a Heartless named Ansem. He fights using ethreal blades, a variety of magical attacks, and can also control Sorcerer Nobodies.
His first appearance was in Kingdom Hearts: Final Mix, although his actual name was not yet given, and the game referred to him as the "Enigmatic Man." If the player returns to Hollow Bastion after all mandatory boss battles are fought, the player will run into Xemnas, who then says he will test Sora. A small hint as to who he is is given when Goofy asks him if he's Ansem. The player will then engage him in a fight that is arguably the hardest fight in the game. If the player succeeds in beating him, Xemnas will tell Sora he looks forward to seeing him again. He also makes a short appearance in the secret ending of the same game.
Xemnas plays no significant role in Kingdom Hearts: Chain of Memories, although the player may assume the cloaked figure in the beginning of the game is Xemnas if they've played Kingdom Hearts: Final Mix first. This figure, however, is later revealed to be Marluxia, the eleventh member of Organization XIII, and the main antagonist of the game.
Xemnas plays a major role in Kingdom Hearts II, as he is the main antagonist of the game. He makes his first appearance with his face obstructed by his hood in Roxas' dreams, where Roxas gets flashbacks of his past in the Organization. He then later reveals who he is in Hollow Bastion, and explains what his plans are. The player will then not encounter Xemnas again until the final area of the game, when all other Organization members have been defeated.
Sora will first fight Xemnas in a solo battle very similar to the battle with "Enigmatic Man" in Kingdom Hearts: Final Mix. When he is thought to be defeated, he absorbs the power of Kingdom Hearts and takes up a stronger form, after which he separates Sora and Riku from the rest, who will then have to fight him alone. Once his stronger form is defeated, he will take up a human form again, this time in a silver cloak, and creates a clone of himself. He finally meets his demise by a combo attack from Sora and Riku.
During the events of Kingdom Hearts: 358/2 Days, more of Xemnas's role as leader of the organization is revealed. The game depicts how he finds Roxas, who begins the game as a mentally blank slate, and grants the Nobody his Organization name before taking him in as the thirteenth member. Xemnas is otherwise seen only a relatively few times over the course of the story, as Saix, his second in command, is in charge of handing out the day-to-day mission assignments to the other Organization members. Like the rest of the Organization, Xemnas is a playable character in the game's multiplayer mode.