What is significant about x in organization xiii

xiii is the roman numeral for thirteen and their names are anagrams of their Somebodies name with an X added. For example: R O X A S = Sora with an X added.
I've compiled a list of what i reckon/know their sombodies name

Xemnas=Xehanort pretending to be Ansem the wise X E M N A S = Ansem
Xigbar=Braig
Xaldin=Dilan
Vexen=Even
Lexaeus=Aeleus
Zexion=Ienzo
Saix=?
Axel=Lea (pronounced L E E)
Demyx=Dyme
Luxord=Durlo
Marluxia=?
Larxene=Arlene
Roxas=Sora
Xion=?