Aqua

Aqua (played by Kate Mara) is one of five protagonists in the prequel Silver's Kingdom Hearts: Birth By Sleep. 
She is a very young and beautiful Keyblade wielder who, along with her friends Terra and Ven, dreamed of becoming a Keyblade Master. She is the only one of the three to become a Keyblade Master. Aqua started off as Master Eraqus' apprentice at the Land of Departure where she and Terra trained to become a Keyblade Master. One day, Master Xehanort arrived and left a new apprentice to work with name Ventus. Aqua and Terra welcomed him with open arms, but Ventus fell into a coma when he was asked questions. Aqua decided to look after Ven in case he woke up, until one day he finally did. 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. One day, Terra and Aqua were put to the Mark of Master Exam, and Aqua becomes a Keyblade Master while Terra failed for not being able to control the darkness within him. After Ven and Terra left the Land of Departure, Aqua was ordered by Master Eraqus to bring Ven back and to keep an eye on Terra if he would get to close to the darkness. During her travels, Aqua found herself in Radiant Garden, where she met young Kairi, and unintentionally performed the Keyblade inheritance ceremony. After a series of events that started to drive the three of them apart, Aqua eventually hears from Yen Sid that Eraqus was struck down by Xehanort and Terra. She eventually reunited with Terra and Ven at the Keyblade Graveyard, where a battle between the three and Master Xehanort and Vanitas took place. During it, Aqua fought Braig and Vanitas (who had taken control of Ventus' heart), and managed to beat them both and destroy the χ-blade with King Mickey's help. But the battle caused Ventus' heart to go into a sleep. Aqua took Ventus' comatose body back to the Land of Departure (which had been destroyed by Master Xehanort), and used her master's Keyblade to transform the land into Castle Oblivion. She left Ventus' body in the Chamber of Waking and promised to be back to wake him up. Aqua found Terra at Radiant Garden, but his body was now the vessel of Master Xehanort's heart. She tried to bring Terra back to his senses, but Xehanort stabbed himself with his own Keyblade, and falls into a portal to the Realm of Darkness. Aqua dived in after him, but realizing she would be unable to save them both, sacrificed her armor and Keyblade and sent him back into the Realm of Light, while she remained behind, waiting to be rescued.