Aqua

Aqua is a main protagonist in the game Kingdom Hearts Birth by Sleep. From the view of the story, it is revealed she is a Keyblade Wielder before Sora. Aqua, along with Terra and Ventus, fights against Master Xehanort and Vanitas. She is in the middle of the three Keyblade Warriors when it comes to age. Like Terra, it is her dream to become a Keyblade Master.

While Aqua means "Water" in Latin, Kairi's name in Japanese means "Sea." Any connection between the two is unconfirmed, however similar name meanings also apply with Riku and Terra and with Sora and Ventus.

Kingdom Hearts Birth by Sleep
After Terra and Ventus, Aqua was the last apprentice to be sent by Master Eraqus to seek out Master Xenahort and his cohort Vanitas who had previously disappeared. Aqua is known to be a level-minded person, but often worries about her friends. She takes the Master Qualification Exam along with Terra, and becomes a Keyblade Master. After fighting Braig and then Ventus under Vanitas' influence, Aqua loses consciousness and she ends up at Yen Sid's Tower. After creating Castle Oblivion to keep Ventus's sleeping body safe, Aqua tracks down Terra to Radiant Garden, only to find him as Xehanort.

Failing to bring Terra back to his senses, Aqua fights and defeats Xehanort, who would have fallen into darkness if Aqua hadn't saved him by sending her armor and Rainstorm Keyblade with him out of the darkness, the two items placed in the Chamber of Repose, a room constructed by the six apprentices of Ansem the Wise under Radiant Garden that is visited by Xehanort in his attempt to find the Chamber of Waking. Aqua ends up in the Realm of Darkness, and twelve years later (after the events of Kingdom Hearts II), she meets Ansem (who is alive and in a Black Coat) and learns about the chain of events that occurred in her absence and about Sora, the new Keyblade wielder. Knowing that Sora will rescue her friends, she utters Sora's name with renewed hope.

Kingdom Hearts II
Aqua's armor and Keyblade were briefly seen in Kingdom Hearts II Final Mix. They reside in the Chamber of Repose, a room constructed by the Organization under Radiant Garden for an as of yet unknown purpose. Xemnas apparently visits the armor often, calling it "friend" and talking to it for long periods of time. This has piqued the curiosity and suspicion of Xigbar, who often eavesdrops on his conversations, and once shared this story with Zexion, stating that one time he heard "another voice" respond to Xemnas. In The World That Never Was, Sora encounters Xigbar, who mentions "other Keyblade bearers", referring to Terra, Aqua, and Ventus.

Abilities
Aqua is described by Tetsuya Nomura as being "tricky". A magic-based fighter with agility to boot, she's capable of doing cartwheels while attacking with her Keyblade. While she's more than capable physically, her prowess lies with a more magic-oriented fighting style, being versed in all forms of it. On top of that, she seems to utilize her own forms of unique magic, including a form of teleportation in mid-battle and an ability that can freeze enemies in time. Her known Mode Changes are:
 * Magic Wish: Her own exclusive Mode Change; it seems to utilize the power of light to attack enemies, while enhancing her magic capabilities as well.
 * Diamond Dust: An ice-based style that seems to specialize in crowd-control and dealing with multiple enemies.
 * Ghost Drive: Aqua's high speed mode; it allows her to attack enemies at high speeds while using illusion properties and tricky movements to her advantage.
 * Thunder Bolt: A swift mode change that unleashes powerful lightning upon attacks.
 * Fire Blazer: A ranged and versatile mode change that focuses the Keyblade to erupt flames while attacking.
 * Ice Blast: A long-ranged ice-based mode accessed via the game's "ice cream deck commands."

Additional abilities include:
 * Dimension Link: A mode that allows Aqua to summon characters from other worlds to aid her in battles.
 * Shoot Lock:
 * Rainbow Shower: Light in seven colors bursts from the Keyblade and chases after enemies. Press at the right time in the circle to input the command. “An attack with light in seven colors.” “A spiraling light whirlwind, a rainbow attack, again!” This move bears a slight similarity to Sora's Ragnarok technique.
 * Bubble Blast: A giant bubble appears around Aqua and attacks by releasing smaller bubbles that chase after enemies.

Personality
Aqua shows some similarities to Kairi, which she shows by making good luck charms for her friends, and Sora, due to her devotion to her friends. Her personality is serious, kind, strong willed, and responsible, as she is the most reliable of the three. She is always worrying about Terra and Ventus, who she viewed as brothers. She has been shown to be blushing and embarrassed when Zack asks her to a date, and starts shaking her head and arms, attempting to compose herself.

Appearance
Aqua looks slightly like Kairi, albeit older. She has blue eyes like the latter, but her hair is blue and shorter. She wears a black and blue, high-collared halter top, two pink, intersecting belts over her chest, and black shorts. She wears an odd silver badge similar to badges also worn by Ven and Terra, though Terra's is gold. 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 what appear to be black stockings or socks on her legs which reach to about halfway up her thighs, leaving a small area of bare skin on her upper thighs. Aqua also bears two strips of blue cloth that drape over either side of her hips, along with a smaller, white strip of cloth tied around her waist, draped in the same manner. Finally, she wears pointed, armored, silver boots 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 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 Ven 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.

Trivia

 * The Japanese Birth by Sleep website's startup screen refers to Aqua as the "Dancing Waters Forming Bonds" (絆繋ぐ舞水).
 * Aqua is chronologically the first female character to wield a Keyblade.
 * The badge on Aqua's chest slightly resembles the symbol of the Heartless and the symbol of the Nobodies turned upside down. The same badge is on Terra's belt (though his is yellow), Ven's chest and Master Eraqus's belt, and is also the keychain of Master Eraqus's Keyblade.
 * In Aqua's "Magic Wish" Mode, she can control her Keyblade using telekinesis, similar to Sora's Master Form and Final Form. She also shoots "Magic Bullets" similar to Sora's Wisdom Form.
 * Aqua is voiced by Megumi Toyoguchi, who previously voiced Paine in Final Fantasy X-2 and Kingdom Hearts II.
 * Aqua is the only known female Keyblade Master. Not quite in Kingdom hearts 2 Kairi is given a Keyblade by Riku in his Ansem form.
 * Oddly, Aqua's appearance in the secret ending is near enough the same as in Kingdom Hearts Birth by Sleep, despite the 12-year time difference. The reasons for this are unknown, though it could be caused by the effects of being in the Realm of Darkness, as Ansem's appearance is also unchanged over the same period of time.
 * Aqua is one of only four characters in the series to call King Mickey by only his name. Others include Yen Sid, Ventus, and Riku.