Unknown

The Unknown is an unidentified character introduced in the North American and European releases of Kingdom Hearts Birth by Sleep, and the Japanese exclusive Kingdom Hearts Birth By Sleep Final Mix. Its boss battle is unlocked when the player defeats Vanitas's Remnant, then loads a save file in which they can go to Land of Departure. Despite this odd situation, its scene is within canon, and is considered to occur at the save file's point in the story. Defeating the Unknown grants the player a unique Keyblade known as "No Name". The Unknown's gender is unclear, though it has a fairly masculine build, as its body is completely covered by a hooded, black coat. It is a tall humanoid and never speaks at any point. It wields two swords that bear a striking resemblance to Xemnas's Ethereal Blades. The blades are made of light blue energy and the black handles are somewhat reminiscent of the Terminus and Ruination Ethereal Blades. The Unknown's name reflects how little is known about it.

http://kingdomhearts.wikia.com/wiki/Mysterious_Figure

Hardest boss in the series now. Vanitas Remnant rivaled the Lingering Sentiment and this dude is way harder. Ah did mah math