Knight

The Knight is a character who appeared in Muscle Woman. He apparently built "Castles" in the creek bed in the Park. It was Mordecai and Rigby's job to get rid of him (originally Muscle man's job). Later, when Starla was destroying the Park, the Knight stepped out and opened up a teleportation hole, and headed back to a place that seemed to have a real castle, making it easy to believe he was a real Knight and just visiting.