Leatherhead is as close to a "cousin" as anything the Turtles have met so far. As a young pet alligator, he was flushed down the toilet by concerned parents. The Kraang captured him and experimented on him. He escaped their clutches and, in the process, took the power cell that allows for transit through the portal connecting Earth and Dimension X.
His martial arts skills are not as highly developed as the Turtles' but his massive form and ability to take a beating makes him a great ally for Team Green!