xemnas - yeh
xigbar - uh-huh
xaldin - uh-huh
vexen - yea-huh
lexaeus - mhm
zexion - indeed
saix - okay, now Saïx has that "i" with the double-dots over it ï, so I assume it has more importance which means that Sai would be discounted. Then again, this could just be Saïx's way of escaping his former identity and since no one else uses the letters in their names properly, he'd be unique.
axel - lately I've started to think that l is interchangeable with r, so Axel could be Era.
demyx - a tough one. DEMY makes a lot of weird words like Edym and so forth.
luxord - same as above. LUORD makes weird words like Drolu.
marluxia - actually, I just don't really care. *shutter*
larxene - Arlene? No wonder she joined XIII. lol