Young Xehanort

Young Xehanort (also called "The Unknown" and the "Mysterious Figure") is a mysterious character, the identity of whom is actually Master Xehanort (albeit at a younger age). He first appeared in the North American, PAL, and Final Mix versions of Kingdom Hearts Birth By Sleep, in which he is fought as a super-boss, before serving as the central antagonist in Kingdom Hearts 3D: Dream Drop Distance. He is voiced by David Gallagher in Birth by Sleep and Benjamin Diskin in Dream Drop Distance.

Young Xehanort is the original Xehanort and was given the power to travel through and manipulate time itself, to help Master Xehanort accomplish his goal of containing his essence in thirteen different vessels.

History
Pre- Birth By Sleep Young Xehanort lived on Destiny islands until his future Heartless, Ansem, time travelled back in time to meet him, Ansem gave him the power to control time and sent him to gather all of their incarnations. Xehanort succeded in doing this and later returned to his time period after Dream Drop Distance to live the life he was fated to.

Birth By Sleep
At an unknown point before Dream Drop Distance Young Xehanort traveled to The Land of Departure after his future self had destroyed it and confronted Terra, Ventus, and Aqua separately after which he disappears without a trace.

Dream Drop Distance
Xehanort travels to the present and has several interactions with Sora and Riku. His purpose during this time is to monitor Sora and ensure he follows a path where he can be controlled. After Sora's battle with Xemnas, Young Xehanort appears and explains that Sora's Heart has already begun to sink into the Darkness and his own origins, ending with stating that since Sora's first encounter with Ansem during the original Kingdom Hearts everything that had happened was unavoidable.

Young Xehanort later appears before Riku when the latter attempts to retrieve Sora. Mickey then appears and uses a Stopza spell to freeze him, however he gets possessed by his elderly self and fights Riku. After their battle Master Xehanort appears and explains the machinations of his plans. After failing to possess Sora, the true Organization XIII's members, including Young Xehanort, return to their orignal timelines.

Kingdom Hearts III
Young Xehanort is set to appear in Kingdom Heart III.