birth by sleep
Destiny is never left to chance.
Kingdom Hearts: Birth by Sleep is a prequel to the main series, and set ten years before the events of the original Kingdom Hearts. As such, it contains a lot of backstory for the universe as a whole as well as details about Xehanort/"Ansem"'s identity, and is arguably the most important side title. It was released on the PSP in 2010, and contains an improved overhaul of the main two games' combat system. It is also due to be remastered for Kingdom Hearts 2.5, scheduled for the end of 2014. Second, it introduces multiple storylines, and allows the player to play the stories of Terra, Ven, and Aqua respectively, as they visit each world and enhance the storyline. Once all three are complete, the final episode opens up as Aqua sets off to battle Xehanort.
In the Land of Departure, Master Eraqus has three Keyblade wielders under his watchful eye: Terra, Aqua, and Ven. The eldest of the pair, Aqua and Terra take the mark of mastery exam but only Aqua passes; Terra makes a mistake, and due to his distant ties with the darkness, makes use of it. Xehanort, a visiting Master, sees this, and thus begins to take advantage of him. Terra leaves the land in an attempt to prove that he should have been made master, followed by Ven, followed by Aqua who is sent with the mission to get them to return. The three friends struggle to keep their friendship alive, despite the promises to each other, and Terra gets all the more trapped.
Eventually, things come to a head when Terra kills Eraqus. Aqua later asks him if he did it, which he says he did. Sweet, innocent Ven asks his two friends to kill him, in order to stop Vanitas, to stop the Unversed. It's at this point Xehanort challenges the three to a final battle in the Keyblade Graveyard. Terra falls and becomes Xehanort's vessel, Ven is reduces to a comatose state having lost a chunk of his heart once again, and Aqua is left to pick up the pieces. Even as she confronts Xehanort and places her final gambit on Terra overcoming his shackles, she too is dragged into the Darkness, away from her friends and the slumbering Ven in Castle Oblivion.
In summary, Birth by Sleep is rather heartbreaking.
Still, the Wayfinder Trio also get to meet their counterparts in the Destiny Trio. Thanks to Aqua protecting her, we find out how Kairi has inherited a Keyblade, and also how she came to Destiny Islands.