Young Xehanort

The Unknown is an enigmatic figure with undefined ties to Master Xehanort. He was introduced as an optional boss in the North American, PAL, and Final Mix versions of Kingdom Hearts Birth by Sleep, and he reappears throughout Kingdom Hearts 3D: Dream Drop Distance as an obstacle to Sora and Riku's Mark of Mastery exam.

Kingdom Hearts Birth by Sleep
After defeating the Vanitas Remnant, Terra, Ventus, and Aqua each separately return to the now-ruined Land of Departure, where they encounter the Unknown. The hooded man draws his weapons and attacks, but after an intense battle, the mysterious figure walks away and disappears.

Kingdom Hearts 3D: Dream Drop Distance
In the Jump Festa 2012 trailer, the Unknown encounters Sora and Riku in La Cité des Cloches and Traverse Town respectively. In the former, he is accompanied by Vanitas.

In the February 2012 special trailer, the Unknown appears before Sora and Riku across various worlds, and Sora and Riku prepare to fight each time they encounter him. Unlike Riku and Sora, his attire doesn't change when he visits The Grid. At one point in La Cité des Cloches, he is accompanied by Ansem, Seeker of Darkness, and in the World That Never Was, by numerous hooded figures.

Appearance
The Unknown resembles Master Xehanort in his youth, and wears a Black Coat. When he first appears in Kingdom Hearts Birth by Sleep, he wears the hood up. He wields two weapons that bear a striking resemblance to Xemnas's Ethereal Blades, but appear to have hilts, which many of Xemnas's blades do not. The blades are made of light blue energy and the black handles are somewhat reminiscent of the Terminus and Ruination Ethereal Blades. The blades can be combined into a dual-blade, and extend to be lashed out like a whip. The Unknown wields these weapons with a reverse grip, similar to the unorthodox way Ven wields his Keyblade.

In Kingdom Hearts Birth by Sleep, the Unknown's body model is a copy of Saïx's, though it is unknown whether this has any deeper meaning.