theory

[Togo]

i think aqua is the fourteenth member.I think they asked her to join them but as a sort of spy, that is why they did not change the name of the organization.They could of sent her to spy on Terra and ven but she grew to like them. then she leads them into a trap and the old man and his apprentice tried to kill terra and ven, that could be the reason why aqua did not fight.After seeing ven frozen she felt guilty.I dunno its the best i could do.

[Kairi]

i think aqua is the fourteenth member.I think they asked her to join them but as a sort of spy, that is why they did not change the name of the organization.They could of sent her to spy on Terra and ven but she grew to like them. then she leads them into a trap and the old man and his apprentice tried to kill terra and ven, that could be the reason why aqua did not fight.After seeing ven frozen she felt guilty.I dunno its the best i could do.
I've always thought that she was a spy too... She betrayed the Organisation and now she's excluded from it for so many reasons I don't have enough room to type. lol. I don't know why, but I've pretty much had the same theory as you when I first heard about her. :) But I hope we get something more about Aqua soon... nothing at all just makes the theory mill work into overdrive.

[Darkecogir]

I don't think so, because BBS is apparently 10 years before KH1 which would mean Aqua would have to be 10 years older and the 14th member looks around Roxas's age

[Ashen Lionheart]

And since she has a Body, she should age at the normal rate. Further, I think the while "Aqua is the 14th Member" thing is just everyone attatching the one possible female name we actually know to the enigmatic female character. I'm certain it's a new character. Maybe it's an old characters Nobody. But the person herself is new.