Aqua is one of the main protagonists of Kingdom Hearts: Birth by Sleep. She is one of the Keyblade Wielders before Sora. Like Terra and Ventus, it's her dream to become a Keyblade Master, a dream she alone was able to accomplish from the three.
Aqua joined Runo and her friends in a battle with the early shadow force. Marik took over her mind and used her against her newly founded friends. Runo freed her and Aqua went after Marik for revenge, but Etna stepped in and killed her. A while later, when Runo and Gabriel make a wish to bring their fallen friends back, she comes back. before she died, she and Amy trained their magical powers.
The Protectors of the Multi-Universe
Aqua appears here as the apprentice of Vincent van Ghoul. They both will tutor the Protectors in their next lessons.