Young Xehanort

Young Xehanort, referred to as the Unknown (謎の男 Nazo no Otoko?)  and the Mysterious Figure 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 one of the main antagonists in Kingdom Hearts 3D: Dream Drop Distance.

This version of Xehanort is Xehanort as a teenager and was given the power to travel through and manipulate time itself to help his older self accomplish his goal of containing his essence in thirteen different vessels.

Pre-Kingdom Hearts Birth By Sleep
The Young Xehanort lived on Destiny Islands until the day when his future Heartless arrived and told him of the future. Ansem then gave his younger self the power to time travel and collect all of their other incarnations. Xehanort did this and eventually returned to his own time.

Kingdom Hearts Birth by Sleep
As Terra, Ventus, and Aqua return separately to Land of Departure's remains, they turn around when they feel a presence and find a hooded figure who challenges them to a battle. After the battle, they seem to have lost sight of the mysterious figure, and prepare to walk away when they spot him again, but he only walks away in the direction of the castle and disappears, fading out of sight.

Kingdom Hearts 3D: Dream Drop Distance
Xehanort's youthful self is revived along with all of the other Xehanorts and appeared numerous times to Sora and Riku as they traverse the Realm of Sleep and awaken several worlds previously destroyed by Ansem. During these appearances, Xehanort would taunt the heroes as he recruited other versions of himself throughout time and space to complete a new Organization XIII under Master Xehanort's orders. The original Organization was meant to recruit thirteen strong beings to be used to make clones of Master Xehanort through the process of transferring portions of his heart into their bodies, but the first generation of Nobodies consisted of many failures. A majority of the original members were either traitorous to the Organization, were too weak for their purpose, or were simply distrustful. Young Xehanort then embarked on his mission to find suitable replacements; various versions of himself, throughout time. To this end, Xehanort recruited Ansem and Xemnas to the new Organization.

As the story progresses, it is revealed that Ansem once traveled back in time and set this younger version of his former incarnation on his journey as a fail safe should Xemnas' Organization XIII prove to be useless in his plans. After Sora fights Xemnas which damages the former's heart, Xehanort puts him into a deep sleep, planning to use him as the final vessel. Sora is taken to the Organization's former headquarters in The World that Never Was, while Riku and King Mickey attempt to save him. Xehanort impedes their attempts and engages Riku in a brutal fight, where Riku comes through victorious, but Master Xehanort returns immediately after the fight to assume control over his new army. Young Xehanort took his place amongst the ranks under his older self's rule and escaped with all his counterparts, once Riku, Mickey, Lea, Donald, and Goofy prevented Master Xehanort from transferring a portion of his heart into Sora to make him his final vessel.

Appearance
In Birth By Sleep, Young Xehanort's body is completely covered by a hooded Black Coat with a fairly tall and masculine build. He never speaks at any point of the game.

In Kingdom Hearts 3D: Dream Drop Distance, his face is revealed to be identical to the youthful Master Xehanort from Birth By Sleep with silver hair, dark skin, an adolescent face and golden eyes.

Young Xehanort's body model is a copy of Saïx's. It may or may not have a deeper meaning as Saix/Isa has not transformed into a full clone of Master Xehanort yet.

Abilities
Young Xehanort is an incredibly powerful being. He was granted the power of space-time manipulation by Ansem, Seeker of Darkness, which he uses to great effect; for example, he recovered from the effect of Mickey's most powerful Stop spell far faster than any of his comrades. His power over time is such is that he can manipulate the flow of battle, performing such feats as slowing his opponent or reversing the flow of time to regenerate health. Like his older self, he is a powerful magic user, able to cast powerful Fire and Blizzard spells, teleport around the battlefield, throw Riku and Mickey with telekinesis, and fire magical bullets similar to those used by Sora's Wisdom Form.

In Kingdom Hearts Birth by Sleep, he is immune to all status effects and possesses incredible speed and agility. Additionally, he is able to duplicate himself, summon pillars of fire, use a countless number of lasers and turn himself invisible. He also has access to a plethora of the numerous protagonists' most powerful attacks, such as Meteor, Mega Flare, Raging Storm and Tornado. In Kingdom Hearts 3D: Dream Drop Distance, he can enter the World of Sleep, summon Dream Eaters, open Keyholes and the Corridors of Darkness.