Young Xehanort

Young Xehanort (also called "The Unknown" and the "Mysterious Figure") is a mysterious character, the identity of whom is actually Master Xehanort (albiet at a younger age). He who first appeared in in the North American, PAL, and Final Mix versions of Kingdom Hearts Birth by Sleep, is a boss in Kingdom Hearts Birth by Sleep and the secondary antagonist in Kingdom Hearts 3D: Dream Drop Distance.

This version of Xehanort is from an alternate timeline 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.

Pre-Birth By Sleep
Young Xehanort lived on Destiny island s until his alternate reality future Heartless, Ansem, time travelled back in time to meet him, Ansem gave him Vanitas' heart and sent him on a mission to find an army of Xehanorts.

Birth By Sleep
Xehanort confronts Terra/Aqua/Ventus at diffrent points in their stories and fights them, later returing to his own time after the battle.

Dream Drop Distance
Xehanort appears with Vanitas in the Cathedral and confronts Sora, he later fights Riku after infusing Sora with darkness. Xehanort/Vanitas is one of the Thirteen Darkness.