Namine isn't an Organization XIII member. With a combination of Jimminy's notes and Ansem's reports, you can deduce that the organization members take their former names and rearrange them with an X. And with some of Jimminy's earlier notes, he says that the name of the members all share a similarity.
Xemnas -
(SPOILER)"Ansem" (he took that name at least)
Vexen - Even
Zexion - Ienzo
Xigbar - Braig
Xaldin - Dilan
Lexaeus - Elaeus
Roxas -
(SPOILER)Sora
You get the idea. It's not stated, but it's very much implied.