Ha, my guess was right: Aqua is, indeed, a female.
EDIT:
ALERT! ALERT! ALERT! ALERT! THE THIRD NAME HAS BEEN CONFIRMED!
I just clicked the web link now, and it has now been confirmed that the name of the third knight is Terra. He is the enigmatic soldier that Sora fought in KH2:FM.
Like Sora, Kairi, and Riku, Ven, Aqua, and Terra are named after natural elements:
Sora means Sky - Ven means Wind
Kairi means Sea - Aqua means Water
Riku means Land - Terra means Earth




