
Xemnas (ゼムナス, Zemunasu) is the Nobody of Xehanort. He is the leader of the Organization, and the main antagonist of Kingdom Hearts II. Xemnas is voiced by Norio Wakamoto in the Japanese version of Kingdom Hearts II, and by Paul St. Peter in the English version. His original self was named Xehanort, but prior to becoming a Nobody he took his master's name, Ansem. He controls Sorcerer Nobodies, and in battle he uses Aerial Blades and manipulates nothingness to phase through solid matter and manifest energy-based shields and projectiles. In Kingdom Hearts Final Mix, Xemnas appears as an optional boss in Hollow Bastion, called "Unknown" (謎の男, Nazo no Otoko, lit. "Unknown Man"). In Kingdom Hearts: Chain of Memories, he is infrequently spoken of by the members of the Organization, who refer to him only as the "Superior".
Personality
Being the founder of Organization XIII and apparently the oldest of Nobodies, Xemnas is truly emotionless and does not even remember what it was like to have emotions. He claims that this allows him to have a clear understanding of the heart, but in reality this has warped his mind into believing that only anger, hatred, and other negative feelings make up a heart.
Not to mention, his lack of emotion gives him absolutely zero remorse over his actions, as well as the loss of the other members of the Organization. The only thing he truly cares about is Kingdom Hearts and his desire for power, and even laments it when it is damaged. Unlike the other members of the organization, Xemnas does not seem to want Kingdom Hearts for a true existence. Rather, he wants it to use its' power to become a god-like entity and take control of the universe. This concept is furthered as Xemnas thinks that nothingness is the source of much power and does not seem to despise it in the slightest. He intended to combine this power with the power of hearts from Kingdom Hearts to become invincible.
In other matters Xemnas speaks with calmness and deep authority. He has a tendency to be overly melodramatic making long-winded speeches, exaggerating his false rage and sorrow with wide sweeping arm movements and hand gestures. While ridiculous in nature, his exaggerations are representations of his lack of emotion. He's very intelligent overall, and knows how to manipulate Sora.
The Superior
Xemnas, as the superior, is the first member and founder of Organization XIII, with all other Nobodies under his control. He is the most powerful member, so much, that Marluxia would not dare to face him even with the support of Axel and Larxene until Sora was completely under his control. Furthermore, Vexen immediately complied with Marluxia's order to eliminate Sora when he made the empty threat that he would report Vexen's failure to the Superior. He also appears to be the Organization's most intelligent member.
Trivia
In the Japanese version, Xemnas was voiced by popular seiyuu Norio Wakamoto, whose last involvement with Square Enix was to voice Al-Cid Margrace of Final Fantasy XII and Dauragon C. Mikado of the Bouncer, as well as Rubicante from the Nintendo DS remake of Final Fantasy IV. The English dub had Xemnas voiced by Paul St. Peter, who voiced Flatski of Brave Fencer Musashi and also the Kyuubi from the english dub of Naruto.
Strangely, Sora doesn't seem to recognize the Organization cloak from his first encounter with Xemnas at Hollow Bastion. It is possible that, since the encounter was an optional boss fight in Final Mix, it shouldn't be considered canon. On the contrary, however, it IS considered canon, seeing as how the cutscene before a boss battle appears when Roxas sees Sora's memories from the first game in the prologue of Kingdom Hearts II. However, this may also be an effect of the loss of memories from his time at Castle Oblivion. This is backed up by the fact that Sora, despite DiZ and Namine's restoration efforts, still has forgotten his abilities, such as his magic.