AppearanceIf you compare the original Birth by Sleep promotional artwork to any finalized artwork, you will see that Aqua's hair design was changed quite a bit. After looking at it for a while it does not seem like the changes should make a huge difference but I personally think they changed Aqua's atmosphere.
The image on the left is from the original promotional artwork while the image on the right is the promotional artwork for Final Mix with Aqua's official design. The difference between the two is pretty obvious. In the original promotional image, her hair spiked at the top of her head as well as extends down the nape of her neck. Aqua's finalized design smoothed the top of her hair out and it is cut very short in the back instead of covering her neck. I am aware that the art style is somewhat different between these two images. However, the images almost look like they are two different characters to me. Aqua looks younger and even more boyish in the original design compared to the Final Mix image. She reminds me of a stereotypical anime protagonist. Her official design gives off an atmosphere more similar to her actual personality: mature and serious. I would guess that they decided to change her hair either because they wanted her to look more like her personality or that they wanted her to look older and more attractive to a male audience. Judging by how many female video game characters seem to be designed, I would bet more on the second reason if anything.
Aqua's design is actually rather conservative compared to the female characters I am used to in a lot of video games. Interestingly enough, I've read that Tetsuya Nomura thought that Aqua's bare back was too revealing in their original design for her outfit and therefore it was changed before the finalized version. Other than her back, shoulders and part of her thighs, Aqua really does not have much skin showing at all. Many female video game characters have a lot of focus drawn to their breasts, hips and butts to make them more "attractive," but all of those areas on Aqua are softened by her design. Her chest is completely covered and though she does visibly have a chest, the material being a darker color makes it less noticeable than if she were wearing a more flamboyant color. She is wearing relatively short shorts, but the cloth she has around her waist covers her butt and hips. Even the nape of the neck, which is traditionally the area on a woman's body that was considered attractive in Japan, is covered by her body suit. Naturally, Kingdom Hearts does not have the same target audience as a fighting game or some other RPGs due to the Disney aspect. I think her outfit is even more conservative than Kairi or Naminé's though, as they both have very short dresses/skirts and have more skin visible.
Aqua's armor is a bit of a different story. Even though all the characters have form-fitting armor, her's seems to accentuate those same features that her normal outfit tends to soften. She has extra armor above her breasts as well as on her hips. To me, it seems like one of those unnecessary additions that makes no real practical sense but is only done to make her more attractive. The extra armor makes her breasts and hips look bigger and therefore gives the illusion that she has more of an hourglass figure than she does. I then wondered why they would bother trying to make her look more "sexy" in armor that she barely has screen time in when her normal outfit made the same features more subtle. When I was planning this section of Stormfall and was thinking about how they changed her hair to make her more attractive, it dawned on me that you can't see her face when she is wearing her armor. Without that atmosphere I spoke of earlier due to her hair and face, she would not be as "attractive" to a male audience unless they accentuated those features. Of course this is all speculation, but since we live in a gaming world that tends to cater towards males, I would not be very surprised.
Overall, I am actually quite fond of Aqua's character design. I was drawn to her from the moment I saw her because I thought she looked awesome. I tend to like the female characters that end up being overly sexualized in video games and anime (mostly because I like the mature characters in general) so it was a nice change of pace to be able to really like Aqua's design. I do think that they made her outfit look more feminine than Terra and Ven's even though they are all basically "soldiers" to some extent, but her outfit isn't overly feminine in a way that it seems unnatural. They pulled Aqua's design off in a way that she was fighting in an outfit that was practical and not overly sexual but yet at the same time still reflected her overall mannerisms and personality. I find that this is becoming harder to come by as video game females seem to have less and less clothes that often are hardly practical for combat as time goes by.