Xemnas
Organization's XIII's Number I. He directs the group as its leader. In truth, he is the Nobody of Xehanort, who was apprenticed to Ansem the Wise. In secret, Xehanort studied the doors and the heart of all worlds, ultimately stealing his master's name, Ansem. When Xehanort became a Heartless, his Nobody Xemnas came into being. Xemnas is using Sora, collecting the hearts released every time a Heartless falls to his Keyblade.
Xigbar
Organization XIII's Number II. When Sora called out to him, thinking he was Riku, Xigbar gave a cult reply. His motives and his meathods remain unclear. Surely he has a greater goal in mind, but what could it be?
Xaldin
Organization XIII's Number III. He sought the Beast's Heartless and Nobody in order to acquire Kingdom Hearts. He was a master of wind who weilded siz lances--but he met his end at the hands of Sora.
Vexen
Formerly Organization XIII's Number IV. He has been eliminated. Vexen, Lexaeus, and Zexion were among Organization XIII's founding members, and there was no love lost between them and neophytes like Marluxia. Vexen opposed Marluxia's plans to take over the Organization, and was consequently destroyed by Axel.
Lexaeus
Formerly Organization XIII's Number V. He once brandished a giant tomahawk, but now he has been eliminated. Lexaeus rejected Marluxia's bid to dominate the Organization, and formulated his own plans--but the plans failed and he met his end.
Zexion
Formerly Organization XIII's Number VI. He has been eliminated. Zexion disliked dirtying his own hands, and relied on his wits to dispose of any rivals--but it was one of his own schemes that wound up destroying him.
Saix
Organization XIII's Number VII. He was in pursuit of the renegade Axel, but for some reason seemed more concerned about Sora's well-being. Saix has no heart, but knows all too well how to injure one. No doubt his poise belies a more savage nature. He also insinuated he knew something about Riku.
Axel
Organization XIII's Number VIII. He commands fire and carries a uniquely- shaped weapon. He has proven to be an odd duck of sorts within the Organization, lashing out against treachery, yet stirring trouble with his own betrayals all the while. He claims he and Sora share some sort of bond, but Sora can't imagine what that might be.
Demyx
Organization XIII's Number IX. He used a type of instrument called a "sitar" to control water...but he wasn't very good at fighting. Demyx was under orders to "liberate Sora's true disposition" while serveying the world of Olympus.
Luxord
Organization XIII's Number X, a gambler who can manipulate time. Having shaken Sora and his companions with wily words, Luxord entertained himself by drawing them into a game-like battle. In Port Royal, Luxord used Heartless and the cursed medallions to conduct experiments on the Organization's behalf and collect hearts.
Marluxia
Formerly Organization XIII's Number XI. He has been eliminated. He plotted an Organization rebellion and tried to seize the power of the Keyblade, but Axel's betrayal led to his destruction.
Larxene
Formerly Organization XIII's Number XII. She has been eliminated. Larxene conspired with Marluxia to turn on the Organization. Larxene was unfeeling and loved nothing more than to bring other people down, but she met her demise when that malice came full circle.
Roxas
Organization XIII's Number XIII, a Keyblade weilder and denizen of darkness. He vanished with a satisfied look after battling Sora--but Sora was the only one who could see him in the first place.