Aqua is a close friend of Terra, as well as an apprentice Keyblade wielder and pupil of Master Eraqus in the Land of Departure. Her relationship with her friends Terra and Ventus, bears a close resemblance to that of the friendship between Sora, Riku and Kairi. Being the only one to pass the test as Keyblade Master, signifies Aqua's maturity compared to her friends.
She's very mother-like towards both of them, trying ever so desperately to keep them both safe from harm. Her wisdom and cautious behavior is also her greatest weakness, as it sometimes becomes a wall that isolates her from her friends. Something that ultimately pushes them further unto the wrong path, and into Xehanort's trappings. She is letting her accomplishments go to her head, despite only wanting what is best and striving to do the right thing. Her devotion to her friends, and her self-sacrifice, bears a stronger connection to that of Sora from a character perspective despite her seeming similarities to Kairi.
Kingdom Hearts: Birth By Sleep
One day, Xehanort dropped off Ventus at the Land of Departure. Terra and Aqua met Ven and realized that he cannot remember anything.
Four years later, Aqua and Terra take part in their final examination as apprentices of Eraqus. She passes her examination and is thus made a Keyblade Master, but Terra, whose heart shows signs of darkness, is forced to remain an apprentice. Later, Eraqus informs Aqua and Terra about the threat of Unversed running amok in the various worlds, and she and Terra both are ordered to travel out and deal with them. Eraqus also pulls Aqua aside and gives her another task in looking after Terra. As she prepares to leave, she also discovers that Ven is running off as well, and is determined to find them both. Aqua hides Ventus away in the Land of Departure, and then uses her power to seal the world away. The Land of Departure now became as Castle Oblivion. Hearing Terra call to her, she sets out to find him.
Aqua finds Terra in Radiant Garden, but as she approaches, Terra attacks her and she realizes that Xehanort is the one in control. Declaring herself Master Aqua, she fights and defeats him. Xehanort, still not in total control over Terra, attempts to drive Terra out using his Keyblade and falls into the darkness. Aqua dives in after him, but is unable to both rescue Terra and return to the world of light. Instead, she sacrifices herself, allowing Terra to return while she becomes lost in a world of darkness. When Xehanort returned to Radiant Garden, Aqua's keyblade and Keyblade Armor was with him. The objects were transported to the Chamber of Repose.
Final Mix Secret Ending (Blank Points)
More than a year after the events of A Fragmentary Passage, Aqua meets Ansem the Wise at the Dark Margin. There Ansem tells Aqua about Sora's adventures, and she once again feels hopeful.
Kingdom Hearts 0.2: A Fragmentary Passage
She then wanders through the darkness for many years until meeting with Mickey Mouse, who had come to the Realm of Darkness to retrieve the Kingdom Key D. After retrieving it, they plan to escape together. However, they soon find Riku fighting against the darkness and Aqua gives up her chance at escape to let the other two get away.