Ansem the Wise had six apprentices; Xehanort, Braig, Dilan, Even, Aeleus, and Ienzo. After Ansem himself cesed his research on the darkness and so forth, Xehanort took Ansem's name and, along with the other five apprentices, continued the research. Through doing this, Xehanort became a Heartless, and the other five soon followed. In addition to becoming Heartless, Nobodies were also created from the apprentices, and these Nobodies, the first to retain their human form, became the founding six members of Organization XIII
Xehanort became Xemnas, "The Superior"
Braig became Xigbar, "The Freeshooter"
Dilan became Xaldin, "The Whirlwind Lancer"
Even became Vexen, "The Chilly Academic"
Aeleus became Lexaeus, "The Silent Hero"
Ienzo became Zexion, "The Cloaked Schemer"
These Nobodies' names are actually an anagram of their original names with an 'X' added... with the exception of "Xemnas", whose name is an anagram of "Ansem" with an 'X' added, due to Xehanort taking Ansem's name. This trait was also used for the later members of Organization XIII, as evidenced by Sora's Nobody, Roxas.
