Members

Prior to the first game in the series, the six former disciples of Ansem the Wise became Heartless and their Nobodies created the Organization. Members are numbered in the order in which they joined, with Xemnas, the Nobody of Xehanort, numbered one as their leader. Each member is given a cloak made of a special material that allows them to pass through the Corridors of Darkness into other worlds safely without being attacked by the Heartless there. Members are given titles indicating both their role in Organization XIII and their personality, as well as their own breed of lesser Nobodies to control, usually suited to their fighting style and unique themed powers. As Nobodies, they lack emotion and morality, and are unable to feel guilt and remorse for their actions. However, most of the members' personalities are based from what memories they have of their original selves. About half of the members were introduced in Kingdom Hearts: Chain of Memories, with the remaining members introduced in Kingdom Hearts II. In Chain of Memories, the series' protagonist Sora and his friends Donald Duck and Goofy are led to a mysterious fortress called Castle Oblivion by members of the Organization, who intend to turn Sora into their pawn by manipulating his memories. Sora's friend Riku also arrives at Castle Oblivion, but in the basement where he encounters other Organization members. Sora and Riku traverse the levels of the castle to reach the top, fighting and defeating several members along the way. By Kingdom Hearts II, the actions of Sora and Riku, along with Roxas' resignation, reduce the Organization to seven members, who seek to use the power of Kingdom Hearts to become complete beings.
The events of the game Kingdom Hearts 3D: Dream Drop Distance reveal that the true purpose of Organization XIII's formation is for Master Xehanort, the true incarnation of Xemnas' human form, to place pieces of his own heart into the bodies of its other members, effectively transforming them into clones of Xehanort so he may use their hearts to recreate the χ-blade. They also reveal that Nobodies such as those in the Organization are capable of developing hearts of their own over time, but are deceived by Xemnas into thinking they lack hearts so they can cooperate with his plans. Though the agenda was not realized for various reasons, a new Organization XIII was founded by recruiting not only the original selves of previous Organization members (Isa and Braig) but also induct various incarnations of Xehanort himself like Xemnas and "Ansem".