Aqua is a character that was originally introduced in the secret video at the end of Kingdom Hearts II: Final Mix+ . She is one of the three protagonists of the Kingdom Hearts prequel Birth by Sleep. Aqua is also a close friend of Terra and Ven, as well as an apprentice Keyblade wielder and pupil of Master Eraqus in the Land of Departure.
At the beginning of Birth by Sleep, 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 shows proficiency with both standard Keyblade attacks as well as a stronger affinity for magic than the other two primary characters. She is also able to form a protective barrier in place of the standard Keyblade parry. In general, her storyline is the most challenging of the three.
If the player collects all twelve of the Secret Reports scattered throughout Aqua, Terra, and Ventus's chapters in Birth by Sleep, the Final Episode will be unlocked. Set after the events in the Keyblade Graveyard, the player once again takes control of Aqua after she hides Ventus away in the Land of Departure, and then uses her power to seal the world away. 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. She then wanders through the darkness for an unknown time until coming face to face with Ansem the Wise.