EDIT: I had posted a much longer and in-depth one here earlier, but something went funky and didn't let me post. So I copy and pasted this one from a write-up I did for my school paper's Videogame Column.
Ok, so we all know from the recently released screenshots and interviews that there IS a Number XIV, and it is a female. All evidence points to...and what's behind door number three? AQUA!
Yes, it is. Here, I'll 'splain it to ya.
Lack of Mention
In all the interviews since the announcement of BBS, 358/2 Days, and Coded, Nomura-san has not mentioned Aqua's name ONCE. All this coincides with the recent revelation of Number XIV. We all know that Aqua is a playable character in BBS, but why hasn't Nomura said her name? Think about it.
Keyblade Choice
The Keyblade she chose in the English SE was Mickey's Keyblade. Perhaps her choice of Keyblade reveals what path she takes after her comrades are defeated by Master Xehanort. Mickey, if we look back, entered the Realm of Darkness, found Kingdom Key Darkside (yes, that is it's confirmed name), and went undercover as an Organization member in KH: CoM and part of KHII. Aqua may have done a little undercover work herself *nudge nudge wink wink*.
Chamber scene in KHII FM+
We all saw the scene in FM+ where Xemnas opens the Chamber of Sleep in the bowels of Radiant Garden, where he has a little convo with our dear Aqua's armor. Signs of schizophrenia? I think NOT! I know it sounds like Xemnas has lost his marbles (and then some), but why would he just visit the Chamber out of the blue? Because. Aqua joined the Organization for a short time then disappeared (due to some mysterious event), and Xemnas was without his lady friend. So he opened the Chamber of Sleep and visited with her remnants. Why armor though? To dawn the Organization persona, she had to do away with her former threads. So when she disppears, he visits the place where she was "awakened" and tossed her past duds.
Organization XIII...not XIV
Simple. The Organization didn't change their name because Aqua wasn't a real Nobody. In fact, she wasn't a Nobody at all. Xemnas awakened her to aid him with the "Roxas problem", since Aqua would know him best (seeing as she and Ven had a little thing goin' on). When her mission was finished with, she suddenly vanished. So they remained Organization XIII because Number XIV wasn't even a Nobody.
Left Alive
Plain and simple. Aqua was the only one left slightly unharmed at the end of the Birth By Sleep SE. Ven froze, and Terra's eyes went yellow. We don't really see what happens to her, besides her being left to hold poor little frozen Ven. She was still alive after that, and she is Number XIV.