The nobodies names are anagrams of thier original names minus the 'X':

Xemnas: Xehanorts Nobody, Xehanort took Ansems name So when he bacame a heartless he was Ansem So: Xemnas = Ansem

Xigbar: Braig, One of Ansems Apprentices.
Xaldin: Dilan, One of Ansems Apprentices.
Vexen: Even, One of Ansems Apprentices.
Zexion: Ienzo, One of Ansems Apprentices.
Lexaeus: Aeleus, One of Ansems Apprentices.

Saïx: Isa
Axel: Lea
Demyx: Edmy
Luxord: Rould
Marluxia: Lumiara
Larxene: Arlene
Roxas: Sora.
Xion: No. I

You get the point........ Xemnas's REAL name (By Psycrowe): Viewer('s) disgresstion is advised!