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. Although his primary obsession is Batman, the Joker has also fought other heroes, including Superman and Wonder Woman.