A playable female protagonist in KH? Unheard of! In my humble opinion, Kairi should have been playable in KHII from the moment she was granted the all-mighty strength of the keyblade, though I recall that she knew how to raise a weapon about as efficiently as one would a baseball bat, so it might have been for the best.

I’ve gone a long ways now in Aqua’s story…and the gameplay is sensational. I’ve always been partial to flashy, dramatic bouts of magic as an offensive tool, but previous KHs never tapped into it with much depth – you could clear an army if Sora spammed Thundaga. There wasn’t much room for creativity and experimentation. Plus, the MP gauge ran out so fast that
it wasn’t practical to rely much on magic alone.
Aqua has none of those restrictions at all; her magical repertoire is limitless- there is so much you can do and it is highly rewarding to mess around with different combinations of magic during battle or melding. You can render any magic to be innovative and functional.
Like ice? Rain it down on your opponents with Diamond Dust. Like fire? Make Triple Firaga and turn those buggers into crisp. The difference in capability of Sora or Donald vs Aqua is as a chicken and a phoenix.

As a character, Aqua is a cut (no wait, make that two cuts) above Kairi and Namine. It’s not the power alone-although that has much to do with it, but she has a ton of courage, determination and a sense of justice.
Kairi and Namine are brave in their own rights, but they were a little too caught up in their personal problems to pursue the greater cause of saving the world. It didn’t help their causes that Kairi has always been stuck or trapped somewhere and Namine had her conscience to deal with. You could say that if Kairi was more powerful, more grown-up and more in tune with the keyblading world, she might be like Aqua.
Overall, Aqua has a lot of high points and not many weaknesses: she’s not naïve like Sora, not solitary like Riku, not powerless like Kairi and certainly not a mixed bag of angst like Roxas (I still really like Roxas, by the way). As a mature, intelligent and responsible protagonist, she’s appealing as a role model.