I think it's a character that hasn't been revealed yet. It's not Namine (Nomura said so), and Aqua would be in her 30s (the 14th member has a connection to Roxas, and she would have more of a connection if she were closer to his age), but other than that it could be anyone. As for her power, I think maybe something that has to do with the light. For example, Xemnas has the power of either darkness or twilight (can't really tell which) and Saix has power over the moon, which would mean he has ties with the darkness. What if the 14th member had power over sunlight? Maybe a mixture of light and fire or something like that...