Batperson is the result of Mark Lilly getting bitten on the genital by a rabid bat boy nicknamed Bruce in the episode "The Dork Knight". The venom from the bite causes him to turn into a bat boy gradually and gives him exceptional hearing, night vision and a supersonic screech. Mark decides to fight crime and makes himself a costume and calls himself "Batperson".
He uses his supersonic screech to fight the cat burglars in New York City. Grimes has a meeting with him and lets him keep on being a vigilante on the condition that he takes a sidekick, Koala Boy, who is actually Doug.
Mark goes to see Callie Maggotbone one night and is discovered by the cat burglars' leader Sabrina Maroney. Sabrina kidnaps Callie and forces Batperson to turn himself in. Batperson and Koala Boy manage to save Callie and catch Sabrina in the end.
Callie cannot stand his screech and orders Doug to bite him which neutralizes the bat boy poison and returns Mark to human again. But Mark's genital needs to be bitten once a day for a year so that he won't relapse.