The Joker is portrayed as a criminal mastermind. Introduced as a psychopath with a warped, sadistic sense of humor, the character became a mostly harmless, comical prankster in the late 1950s in response to regulation by the Comics Code Authority, before returning to his darker roots during the early 1970s (although some of his more comedic characterization was kept for many incarnations of the character).
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 became his criminal sidekick and girlfriend before finally escaping their abusive relationship.