I personally love the entire Birth by Sleep trio. They're a marked improvement over their predecessors of Sora, Riku, and Kairi. Ventus and Terra are basically Sora and Riku with a more believable bond and a more intricate and satisfying backstory. Aqua, on the other hand, is so much of an improvement over Kairi that it's insane.
Kairi is nothing more than a damsel in distress for the first game, an object of affection that causes tension between her two childhood friends, both of whom have feelings for her. An age old story that was only made fresh because it was in an experimental Disney/Square-Enix crossover game. She improves in subsequent entries, but still remains a background character for much of them. She gets her own keyblade and can at least... try... to fight. Overall, though, she's still the weakest link in what is now a huge cast of characters.
Aqua is her complete inverse. She has competency, agency, and is actually playable in her own game. She is essentially the leader of her group of friends, and this shows in that she is the only one to pass the Mark of Mastery exam and become a Keyblade Master. She has the most unique playstyle out of any Kingdom Hearts protagonist with her focus on floaty, magic oriented combat. Not to mention her visual design is killer - an excellent use of blues and relaxing, subdued colors helps to exemplify her calm wisdom, as well as flowing, silky clothes that work excellently with her fighting style.
I love Aqua as a character. She's probably my favorite female protagonist from a Square-Enix game, and one of the rare Kingdom Hearts characters that feels like they have some sort of substance and depth to them.
