He debuted in the first season episode "The Finger of Light," where it is mentioned that he has been trying to start a cult for years, but nobody wants to join it. A nervously energetic sort with a voice and appearance modeled after Peter Lorre, Marlon is unusually ambitious for a Fraggle and sometimes slightly sinister. Following his failed bid to become Ruler of the Rock, Marlon toadies up to the winner, Mokey Fraggle.
He later graciously offers to rig the scoring on Red's feats in "Red's Sea Monster," and is the only one to believe her claim of a sea beast (again, hoping to use it to his advantage).
Similarly, he embraces the vistas offered by Wembley's ability to fly in "Wembley's Flight," asking that he unfurl a banner saying "Elect Marlon, King of the Fraggles."