Xemnas

Xemnas is the main antagonist from Kingdom Hearts II. He is of the Nobody species. He is the master of wielding the element of Nothingness. He is the leader of the notorious group, Organization 13.

Past
Xemnas used to be a man by the name of Xehanort, who was the student of a man named Ansem the Wise in a world called Radiant Garden. Ansem the Wise was the friendly ruler of Radiant Garden, and was intent of freeing the darkness from everyone's heart, although everyone in the universe, save seven princesses, have darkness in their hearts. He and his students, including Xehanort, conducted experiments on darkness. This intrigues Xehanort, and Xehanort does more dangerous experiments behind his master's back. He creates a thing called a Heartless, which is formed when a person's heart is taken, for a lost heart makes a Heartless. Xehanort went under Ansem's name and wrote a very detailed report on it all. Xehanort also breaks the barriers between all worlds, thus letting people be able to travel anywhere. Soon, King Mickey uses a Gummi Ship to fly to Radiant Garden to tell "Ansem the Wise" about a Keyblade, what has the power to open the door. Xehanort then sends a young girl, Kairi, to other worlds via sea in hopes that she will stop the Key Bearer. Xehanort also got Ansem the Wise's other five students involved with his reckless experiments, and eventually, Xehanort discovered that with every Heartless created, an empty shell is left behind and is called a Nobody. Xehanort then transformed himself into a Nobody, along with the other five students. Only Xehanort however, had a Heartless, simply known as "Xehanort's Heartless". Xehanort's Heartless tries to plunge all worlds into darkness by destroying all worlds, while the Nobody half does other things...

Organization 13
Xehanort, now known as Xemnas, is the leader of a group that will eventually become Organization 13, but at the time, only consisted of six members, Xemnas, Xigbar, Xaldin, Vexen, Lexaeus, and Zexion. They eventually recruited the other seven Nobodies, Saix, Axel, Demyx, Luxord, Marluxia, Larxene, and Roxas. Xemnas' plan was now to instead of getting hearts from worlds, he would send Heartless at the Key Bearer, who is a boy named Sora, so Sora would kill them, thus making Kingdom Hearts capable of making Organization 13 whole again, not just emotionless Nobodies. However, Sora and his friend Riku slowly killed off the members of the group one by one, until only Xemnas was left. Sora and Riku finally defeated Xemnas, while ridding the universe of Organization 13.