Aqua's name means "Water" in Latin, while Kairi's name means "Sea" in Japanese. The only connection between the two is that Aqua met Kairi in Radiant Garden and put a protective spell on her necklace, which later brought Kairi to Destiny Islands. Kairi, by accidentally touching Aqua's Keyblade , she inherited the Keyblade from Aqua.
Aqua, alongside Terra and later Ventus, is one of Eraqus's apprentices. Like Terra, Aqua also had a dream of becoming a Keyblade master one day.
One day, Master Xehanort arrived and gave Master Eraqus a new boy to work with named Ventus. Terra calls out to Aqua, telling her that a new apprentice had just arrived. While chatting with Ven, Aqua and Terra asked Ven about why he wanted to train to become a Keyblade master, but this question, as well as a few other questions troubled Ven, so much that eventually he fantied.
Eraqus tells Aqua and Terra that the reason Ven fainted was because he had a loss of memorie. Aqua decided to kept watch after Ven, in case he woke up. Many days passed, but despite Ven being unable to awaken, Aqua would not abandon him. One day, Aqua saw Ven open his eyes and regain consciousness, he finally woke up.
Since that day, Aqua and Ven became close friends and rivals. Days have gone by, and she, Terra, and Ven became close like siblings, sparring with each other, sharing dreams and secrets, Aqua wanted to keep their bond strong, so one night, she shared with Terra and Ven Good Luck Charms, made by her to remember their long lasting friendship.
The next day, she takes the Mark of Mastery Exam along with Terra, and becomes a Keyblade Master, although Terra fails due to having darkness within hos heart. After Terra and Ventus left, Aqua was the last to be sent by Master Eraqus to bring Ventus back to the Land of Departure . But before she left, Eraqus instructs to 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 to close to the darkness, Eraqus needed Aqua to bring Terra back immediately.
Aqua, Ven, and Terra come across many worlds and new people along the way and fight of hordes of Unversed . Aqua later ends up at Radiant Garden with Mickey Mouse and meets Kiri, the Princess of Heart, who touched by accident Aqua's Keyblade in fright, unintentionally performing a Keyblade Inheritance Ceremony . They 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 to help her in the future. Aqua believes that their meeting may not have been by accident.
At one point Aqua and Terra come to Destiny Islands and meet two young boys called Sora and Riku . Aqua sees the strong bond between the two friends and advises Sora to help Riku if he ever falls into darkness. Aqua later encounters Mickey unconscious drifting in space and takes him to the Mysterious Tower , where she hears of Eraqus's death from Yen Sid and goes to the Keyblade Graveyard to meet up with Terra and Ven. There they are confronted by Master Xehanort and Vanitas, his apprentice, and a great battle takes place. After fighting Braig and then Ventus under Vanitas's influence, Aqua loses consciousness and she ends up at Yen Sid's Tower with Ventus, who has lost his heart. She takes Ven back to the Land of Departure and places his unconscious body in a throne. She takes Eraqus's Keyblade (Master Keeper) and transforms the world into Castle Oblivion to keep Ventus safe. She then tracks downTerra's possessed body in Radiant Garden.
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 Keyblade with him out of the darkness. Now trapped in the Realm of Darkness, Aqua is attacked by giant Heartless , but she's saved by Terra and Ventus's Keyblades. Aqua is then reminded of the people she's connected to. With renewed trust in her friends, Aqua continues on.