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 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 ).
 Before Kingdom Hearts Birth by Sleep
Aqua, alongside Terra, was one of Eraqus's pupils. Like Terra, Aqua also had a dream of becoming a Keyblade master.
One day, a suspicious man named Master Xehanort arrived and gave Eraqus a new apprentice to work with, a boy named Ventus. Terra called out to Aqua and told her that a new apprentice had just arrived. Aqua and Terra tried to converse with Ven and asked him about why he wanted to train to become a Keyblade master. The questions troubled Ven to the point that he couldn't take the pressure. He fainted and shocked Aqua and Terra.
Eraqus told Aqua and Terra that the reason Ven fainted was because he had lost his memories. Aqua decided to look after Ven, in case he woke up. Many days passed and, despite Ven being unable to awaken, Aqua did not abandon him. One day, Aqua saw Ven open his eyes and regain consciousness. Overjoyed, she quickly informed a worried Terra and Master Eraqus.
Since that day, Aqua developed a very close relationship with Ventus, acting much like his older sister and becoming very protective of him. Over the years, Aqua, Terra, and Ven forged a strong bond with each other.
 Kingdom Hearts Birth by Sleep
One night, Ven, Terra, and Aqua watch a meteor shower together and she gives them each a good luck charm, and saves one for herself.
The next day, she takes the Mark of Mastery exam along with Terra, and becomes a Keyblade Master, although Terra fails due to his inability to control his darkness. After Terra and Ventus left, Aqua was the last apprentice to be sent by Eraqus to bring Ventus back to the Land of Departure. Before she left, Eraqus wanted to ask Aqua to keep an eye on Terra, to make sure he is alright, and stays safe from the darkness in his heart. If Terra's heart would become too close to the darkness, Eraqus needed Aqua to bring Terra back immediately.
Aqua, Ven, and Terra make many new friends along the way and fight off hordes of Unversed. Aqua later ends up at Radiant Garden with Mickey Mouse and meets the young Princess of Heart, Kairi, who touched Aqua's Keyblade in fright, unintentionally performing a Keyblade Inheritance Ceremony. Aqua and Mickey protect her from a group of Unversed, and in return, Kairi gives Aqua flowers, which becomes the Keyblade Destiny's Embrace. Aqua also places a protective charm on Kairi's necklace to help her in the future, believing that their meeting may not have been a coincidence.
Aqua later meets up with Terra and Ventus, but the reunion is short-lived as they team up to battle a massive Unversed. They manage to defeat it, but things take a turn for the worse. Terra and Aqua begin to argue about Master Eraqus's orders, revealing to Terra that she was sent to watch him. Hurt by this, Terra leaves, and Ventus calls Aqua awful, claiming that she has let her new status as a Keyblade master go to her head. He then leaves to find Terra, as Aqua stays to contemplate.
After meeting Merlin at his house, Aqua goes to the town square and encounters the enigmatic Vanitas, who taunts her by inquiring about Ventus and if he has gotten stronger. When Aqua asks what he means, Vanitas attacks, and the pair fight a vicious battle. Aqua emerges victorious, but Vanitas simply leaves via Corridor of Darkness, making an ambiguous comment about her being his backup plan. Aqua then resolves to stop Vanitas to protect Ventus and Terra. Ventus then appears, asking to go with Aqua. At first she is harsh with him and he appears discouraged. She softens and tells him she doesn't want to put him 'in harm's way.', and departs not long after.
While in Neverland, Aqua encounters Peter Pan and the Lost Boys, joining them on their treasure hunt. They later find their treasure chest, filled with wooden swords and Ventus's wooden Keyblade. Peter Pan tells Aqua that Ventus left the toy there as he no longer needed it. Aqua then senses someone nearby and goes off to investigate. She then encounters Vanitas again, this time with Ventus's cherished Wooden Keyblade. The dark enigma cruelly mocks Aqua's actions and snaps the toy weapon in two. Greatly enraged by this, Aqua battles and defeats him again, but is left exhausted by the fight and passes out. While unconscious, she reminisces about the first starry night she spent with Terra and Ventus. She soon recovers and leaves Neverland.
At one point Aqua and Terra come to Destiny Islands and meet two young boys by the name of Sora and Riku. Aqua sees the strong bond between the two friends and advises Sora to help Riku if he ever falls into darkness; though she initially intends to grant Sora the power to use the Keyblade, she decides against it upon discovering that Terra had already done it for Riku, not wishing to put the two innocent children through the ordeals she and Terra have faced.
Aqua later encounters an unconscious Mickey drifting in space and takes him to the Mysterious Tower, where she hears of Eraqus's death from Yen Sid and heads to the Keyblade Graveyard to meet up with Terra and Ven.
A short conversation with Ventus takes place there, one where he states that if he does become the X-blade, he wants the two of them to 'put an end to me.' Aqua appears horrified, but Master Xehanort makes an appearance and tells about the history of the Keyblade War. Afterwards they attack, and a great battle takes place. Xehanort holds Ven over the edge of a cliff where Aqua can see, he freezes the boy and drops him, however, she manages to catch him. After fighting Braig and then Ventus under Vanitas's influence, the X-blade is destroyed and she is flung into the Lanes Between, unconscious. Mickey finds her and Ventus drifting there and brings them both to Yen Sid's Tower, where it is revealed to her that Ventus has lost his heart. She takes Ven back to the Land of Departure and places his sleeping body in a throne. She takes Eraqus's Keyblade and transforms the world into Castle Oblivion to keep Ventus safe. Afterwards she says her goodbyes to Ven, fondly stroking his hair, and leaves. She then tracks down the possessed Terra to Radiant Garden.
Failing to bring Terra back to his senses, Aqua fights and defeats Xehanort; subsequently, Xehanort stabs himself in the chest with his own Keyblade in an attempt to lock Terra out as a horrified Aqua looks on. As Terra-Xehanort falls through a portal into the Realm of Darkness, Aqua dives in after him, but quickly realizes that she will be unable to save them both. Instead, Aqua sacrifices her armor and Keyblade to save the dark being that had once been her dear friend, sending him back into the Realm of Light while she remains behind. Now stuck in the Realm of Darkness, Aqua is attacked by several giant Heartless and decides to just allow them to destroy her, but is saved by Terra and Ventus's Keyblades. She is then reminded of the people she's connected to. With renewed trust in her friends, Aqua continues on.
As she travels she sees a large circle of light form above her, only for a powerful column of darkness to shoot out of it in front of her. Seemingly unshaken by this she presses forwards and encounters several Shadows, Neoshadows, and Darkballs. After traveling deeper into the Realm of Darkness, Aqua senses something stalking her, and comes fact to face with a large Heartless that attacks her. To prevent it from attacking her she uses a Fire spell, only for it to leap over her. Getting a closer look at it, Aqua determines it is not an Unversed and takes a battle stance to fight it.
After felling the Heartless, Aqua continues walking, eventually coming to a clearing and looking up. Surprised, she recognizes the Castle of Dreams surrounded by clouds of darkness.
 Kingdom Hearts II
Aqua's armor and Keyblade reside in the Chamber of Repose, a room constructed by Ansem's Apprentices under Radiant Garden for an as of yet unknown purpose. Xemnas 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.
 Blank Points
Aqua ends up at the Dark Meridian, where she meets Ansem the Wise, who was cast into the Realm when his machine exploded. He reveals that in the Realm of Light, it has been almost eleven years, and he explains the events that have occurred in Aqua's absence, including the actions of Sora. Knowing that Sora will rescue her and her friends, she sheds a single tear, while uttering Sora's name with renewed hope.
 Kingdom Hearts coded
Aqua is alluded to by Data-Naminé as one of those connected to Sora's heart and need his help, appearing as one of Data-Naminé's mental images. Later on, after Mickey informs Yen Sid that they have located Ventus's heart, Yen Sid concludes that Terra is the only one left to find, implying that they have already located Aqua.
 Kingdom Hearts 3D: Dream Drop Distance
Mickey mentions Aqua when he suggests that he help Sora and Riku inside the Dreaming Realm by following his heart's path, as Aqua told him long ago.
When Sora arrives at The World That Never Was, he is caught by the Organization and placed within a nightmare in which he chases after memories of Riku and Kairi. When he catches up to them at the Contorted City, the phantoms suddenly take the form of Terra and Aqua, while Sora himself takes on Ventus's appearance. Sora is confused, but the phantoms grasp his hand before turning back into Riku and Kairi and walking off again. As Riku attempts to wake Sora on his own side of the dream, Sora recalls when Aqua came to Destiny Islands, and nearly wakes up.
However, Ansem appears within Sora's dream at the last second and thrusts him back into the nightmare. Xigbar and Xemnas reappear within the nightmare to taunt him and lead him into despair by explaining the inevitability of their plan and playing on his insecurities, but a defiant Sora proclaims that while he may not have been chosen by the Keyblade, he will always be proud to be there for his friends, including Aqua and all the others.
Aqua has several similarities to Kairi, which are shown by making good luck charms for her friends. She also has a devotion to her friends similar to that of Sora's, for she did everything in her power to protect Ventus from harm and even sacrificed herself to save Terra. 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 are two of her closest friends. She has been shown to be blushing and embarrassed when Zack asks her to a date, shaking her head and arms in an attempt to compose herself. She seems rather modest, as despite becoming a master she still referred to herself as a trainee. The only time she referred to herself as "Master Aqua" was toward the end, when she fights the possessed Terra.
Aqua has blue eyes, 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 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 Sleep's 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.
 Keyblade and fighting style
Aqua is described by Tetsuya Nomura as being "tricky". In terms of physical ability, she is the most balanced out of the three main protagonists of Kingdom Hearts Birth by Sleep, with decent strength, speed, agility, and range. She is neither the slowest nor the fastest of the trio, but her slender frame makes her a difficult target. Unlike Terra and Ventus, she blocks using Barrier, a costless version of the Reflect spell, effectively giving Aqua a 360° defensive zone.
Aqua has very few exculsive attack commands, consisting of Barrier Surge, Wishing Edge, and her most powerful attack command, Magic Hour. Her personal finish commands include the Magic Pulse series, Heat Slash 2, Ice Burst, and her ultimate finisher, Teleport Spike.
While she's more than capable physically, her prowess lies with a more magic-oriented fighting style, being versed in all forms of it. Aqua is second only to Master Xehanort in terms of magic amongst Keyblade users, displaying superior skill to both Terra and Ven as well as later Keyblade users such as Sora and Riku. Aqua has a wide variety of Magic Commands. She has her own special variations of the Fire, Ice, and Thunder spells. Fire spells include Triple Firaga, Firaga Burst, and Raging Storm. Ice spells include Triple Blizzaga, Glacier, and Ice Barrage. A Thunder spell she can use is Thundaga Shot. She can also use Seeker Mine, and has a few unique magnet spells like Energy Magnet, Munny Magnet, and D-Link Magnet.
Beside a variety of these abilities usable by all three or two of the protagonists in Kingdom Hearts Birth by Sleep, Aqua has access to two unique Command Styles and three exclusive Shotlocks. Her first exclusive Command Style is Spellweaver, which hovers her Keyblade in midair, swiftly attacking enemies while enhancing her magic capabilities as well. Her second style is Ghost Drive, a second level style which allows her to warp around enemies, attacking them at high speeds while using after-image illusions and tricky movements to her advantage.
Aqua has access to the evasion technique, Cartwheel, as well an offensive variant, Firewheel. She can also use Doubleflight, allowing her to jump once and jump again in midair. Lastly, she can use Teleport, allowing her to warp behind an enemy, dodging its attack.
Aqua becomes a D-Link for Terra and Ventus, after they received the Wayfinder, Aqua made for the three of them. While in link with Aqua, the player can utilize powerful magic attacks.
Aqua's exclusive Keychains generally share a water-theme and are primarily focused on magical strength; in fact, her "Brightcrest" is the most magically-powerful Keychain in the game. Her Keyblade is of medium length, between those of Ventus and Terra. Aqua starts off with her personal "Rainfell", but after visiting Destiny Islands, she obtains a more powerful version of it named "Stormfall". However, after falling into the Realm of Darkness, she sacrifices her Keyblade and Keyblade Armor to save Terra, and is forced to use "Master Keeper", her former Master's Keyblade, which she had retrieved earlier. Small petals appear when she summons her Keyblade.