Dirk Mullen is the soulless Rewind in UnBound: Rewinds.
Dirk, like his Rewind brethren, was made entirely out of unwound human parts. The only difference between him and the others was a major difference: Dirk had no soul. This becomes evident through many things, such as him seemingly just picking random words when he is told to pick a name, and not understanding compassion very well, and, most notably, having "nothing" in his eyes.
When Dirk meets Keaton, he notices that they each have an umber arm from the same Unwind. He tells Keaton that it is the "hand of my hand," claiming that they are brothers, which Keaton denies, not wanting to associate with the soulless Rewind.
When Keaton talks to a girl on the other side of the fence, Dirk scares her away, screaming about prom. Later, Dirk tries to get Keaton to sneak out with him, and has to resort to knocking him out, leaving a wound on his forehead. Dirk finds the girl he had scared off earlier, and attacks her, screaming about prom, and that he would "make her like it." The girl hits him, leaving a wound on his forehead.
When Keaton tries to find Dirk, he is ambushed by the people on the island, as he matches the description of "one in her arm and a wound on his forehead," even though the girl was describing Dirk. The girl eventually finds out, and convinces the people that they had the wrong Rewind. Dirk rides off on a motorcycle, trying to escape.
Eventually, Cam, Una, and Keaton find Dirk, holding a gun. Keaton is the only one Dirk would let near him without hurting him, so Keaton approaches and kills him. There is a funeral for Dirk, but only Cam and Una attend.