This is Aqua's Scenario in Kingdom Hearts Birth by Sleep: The Lost Chapters. Enjoy. (Note: There is no shipping, period.)
Many years ago, there was a war between the forces of light and darkness that resulted the seperation and isolation between worlds. This war was known as the Keyblade Wars. Years later, in a world known as the Land of Departure, three Keyblade wielders and close friends-Terra,Aqua, and Ventus (more commonly known as Ven)-train to become Keyblade Masters under Master Eraqus. To achieve this rank, apprentices must take the Master Qualification Exam once they come of age. Terra and Aqua take this test together, but while Aqua passes the test, Terra is deemed unfit to become a Keyblade Master due to the abundant darkness in his heart.
Afterward, a group of mysterious creatures called the Unversed begin to surface in other worlds, and another Keyblade Master, Xehanort, disappears without a trace. Terra is sent to destroy the Unversed and find Xehanort, who had earlier encouraged him to embrace the darkness in his heart if he desires the strength to become a Keyblade Master. Ven, meanwhile, is goaded into following Terra by Xehanort's mysterious apprentice Vanitas, against Eraqus's wishes. Aqua is dispatched by Eraqus to survey Terra and keep him from falling into darkness, as well as to bring Ven back to the Land of Departure.