In his comic book appearances, the Joker is portrayed as a criminal mastermind. Introduced as a psychopath with a warped, sadistic sense of humor, the character became a goofy prankster in the late 1950s in response to regulation by the Comics Code Authority, before returning to his darker roots during the early 1970s. As Batman's nemesis, the Joker has been part of the superhero's defining stories, including the murder of Jason Todd—the second Robin and Batman's ward—and the paralysis of one of Batman's allies, Barbara Gordon. The Joker has had various possible origin stories during his decades of appearances. The most common story involves him falling into a tank of chemical waste which bleaches his skin white, turns his hair green, and his lips bright red; the resulting disfigurement drives him insane.
The Joker possesses no superhuman abilities, instead using his expertise in chemical engineering to develop poisonous or lethal concoctions, and thematic weaponry, including razor-tipped playing cards, deadly joy buzzers, and acid-spraying lapel flowers. The Joker sometimes works with other Gotham City supervillains such as the Penguin and Two-Face, and groups like the Injustice Gang and Injustice League, but these relationships often collapse due to the Joker's desire for unbridled chaos. The 1990s introduced a romantic interest for the Joker in his former psychiatrist, Harley Quinn, who becomes his villainous sidekick.