Quarx is an extraterrestrial boy in the 1987 TV series. He appears in The Starchild episode. He appears to be a small alien creature with the maturity of a preschool child, often declaring that he wants to play and refusing to do things he doesn't want to. If anyone tried to persuade him to do anything, he would declare "You're not my father!"
Quarx has nearly godlike powers, and is responsible for a trail of destruction across many worlds because of his immaturity. After finding him in the wreck of a spacecraft, the Turtles attempted to help and care for him, defending him against the alien bounty hunter Drako. However, they also were faced with an alien armada that would destroy the planet if Quarx was not handed over to them.
Eventually the Turtles learned of the experiments that gave Quarx his powers, and that he is choosing not to mature because his father is not there any longer. Splinter masqueraded as his father, and told Quarx that it was time to finally grow up.
Upon maturing into his adult form, Quarx also acquired emotional maturity. He used his powers to repair all the damage on Earth that he had caused, and pledged to also fix the problems he caused on other worlds.