Darkling

The Black Costume is a group of Heartless consisting of Players fallen to the darkness. Since they willingly succumbed to the darkness, they kept their sense of self. They are also still able to speak simple sentences, access the Lanes between and fight without drawing power from the Book of Prophecies' fragments.

Kingdom Hearts χ[chi]
When the Player and Skuld reach the clock tower surroundings in Daybreak Town, they are surprised by the Black Costume who attack them for their Lux.