Aqua is one of the main protagonists of Kingdom Hearts Birth by Sleep. From the view of the story, it is revealed she is one of the Keyblade Wielders before Sora. Like Terra and Ventus, it is her dream to become a Keyblade Master, a dream she is the only one of the three to accomplish.
Her name is Latin for "water", while Kairi's name in Japanese means "sea". The only connection between the two is that Aqua met Kairi in Radiant Garden in Kingdom Hearts Birth by Sleep and put a protective spell on her necklace, which later brought Kairi to Destiny Islands. Kairi, by accidentally touching Aqua's Keyblade during this time, also inherited the Keyblade from Aqua. The official Japanese website refers to Aqua as the "Dancing Waters Forming Bonds" (絆繋ぐ舞水 Kizuna Tsunagu Busui).
Aqua has a tall and slender figure, about as thin as Ventus and standing a few inches shorter than Terra. She has blue eyes and medium-length blue hair, mostly parted to her left. She wears a black and navy blue, high-collared halter top, a black corset with two pairs of white laces and black shorts. She has two pink straps intersecting over her chest, with a silver badge similar to badges also worn by Ven and Terra over the intersection. On Aqua's arms are white bell-sleeves and tan, fingerless gloves. She also wears a small, segmented piece of armor on each of her upper arms. She wears black stockings reaching about halfway up her thighs, leaving a small area of bare skin. There are two strips of blue cloth draped over each of her hips, along with a smaller, white strip of cloth tied around her waist and draped in the same manner. Her silver boots are pointed and armored, with a sharp "hook" on the outer side of each.
By touching the piece of armor hanging on her arm, Aqua will become full clad in her Keyblade Armor. In Birth by Sleep, Aqua's armor is mostly colored in shades of silver, black, grey and blue. Notable additions include a silver breastplate resembling an upside-down crescent moon and a dark-visored helmet. Her helmet, like Ventus and Terra's, bears two prongs on either side, with her silver ones wrapping around the sides and angling diagonally downwards. Her cape is that of a dusty-rose color. In Kingdom Hearts Birth by Sleep, Aqua's armor remains largely unchanged, though she loses her cape and grey becomes a more dominant color than black or blue. She also gains more armor near her upper legs and hips.
During the development for Kingdom Hearts Birth by Sleep, Aqua's outfit was altered, as Tetsuya Nomura considered her exposed back to be too revealing, ultimately resulting in an outfit that is neither too conservative nor too revealing. As well, her eyes and hair were originally darker.
Artwork of Aqua from Kingdom Hearts Birth by Sleeps promotional artwork shows her hair spiking near the back of her head and falling just past her neck, as opposed to the spikes being splayed to the side and the back stopping short of the base of her neck.
Aqua bears some similarities to Kairi and Sora, primarily in her sisterly affection for her closest friends. This is displayed by when she makes lucky charms for them, does everything in her power to protect Ventus (despite his apparent discouragement whenever she tells him to go home), her confidence that Terra will overcome the darkness, and when she strands herself in the Realm of Darkness in order to save him. Aqua is also shown to be very sociable with children, quickly befriending Kairi, Sora, and Riku. Aqua is remarkably calm and dead-set on her goals, and only threats on her loved ones seem to make her lose her composure. Although, like Sora, Aqua is comically insecure in a romantic sense, so when Zack asks her out on a date, she becomes completely flustered and desperately tries to compose herself. She also quickly changes the subject when Hercules asks why her face is red.
Aqua is typically modest, referring to herself as a trainee (only calling herself "Master Aqua" before she fights Terra-Xehanort, even then only as an intimidation tactic) and acting very respectful toward elders and authorities such as Master Eraqus, Queen Minnie and the Grand Councilwoman. However, like Eraqus, Aqua is also strict, stubborn and overprotective, which sometimes makes her seem condescending. She always worries about Ventus because of his age and lack of experience, ordering him to go home whenever she finds him without giving him a chance to prove himself.
However, despite her flaws, when the situation demands it, Aqua's overall skill, determination, intelligence and kindness make her the most reliable and competent out of Eraqus's apprentices, and unquestionably worthy to be a Keyblade Master.
Aqua joined Runo and her friends in a battle with the early shadow force. Marik took over her mind and used her against her newly founded friends. Runo freed her and Aqua went after Marik for revenge, but Etna stepped in and killed her. A while later, when Runo and Gabriel make a wish to bring their fallen friends back, she comes back. before she died, she and Amy trained their magical powers.
The Protectors of the Multi-Universe
Aqua appears here as the apprentice of Vincent van Ghoul. They both will tutor the Protectors in their next lessons.