The Joker is a master criminal with a clown-like appearance, including bleached white skin, red lips, and green hair.
He is a depressed, suicidal career criminal, who so excels at theft and murder that they no longer provide any thrill for him. In the course of a particular theft, he encounters Batman. The encounter jolts him out of his depression; he is inspired and amused by Batman's ludicrousness. Revitalized, he launches into a crime spree to toy with Batman. The conflict between him and Batman becomes so heated that Batman chooses to hand him over to the mob, who wants him dead. Grateful, the mob abducts him and takes him to a chemical plant to torture and murder him. Despite Batman's efforts to rescue him (having regretted his decision), an accident occurs and he is swept away with chemicals, producing the Joker.
The Joker commits crimes with countless "comedic" weapons (such as razor-sharp playing cards, acid-spewing flowers, cyanide pies, and lethally electric joy buzzers) and Joker venom, a deadly poison that infects his victims with a ghoulish rictus grin as they die while laughing uncontrollably. This venom comes in many forms, from gas to darts to liquid poison, and has been his primary calling card from his first appearance. The Joker is immune to his venom. The Joker is highly intelligent and very skilled in the fields of chemistry, genetics, and engineering. He is shown as very computer literate.
After Scarecrow and the Joker team up and kidnap the mayor of Gotham City, Scarecrow turns on the Joker and uses his fear gas to see what Joker is afraid of. To Scarecrow's surprise, the gas has no effect on Joker, who in turn beats him with a chair.
The Joker has been referred to as the Clown Prince of Crime, the Harlequin of Hate, and the Ace of Knaves.
The Joker's victims have included men, women, children, and even his own henchmen. The Joker once gassed an entire kindergarten class. The Joker is reported to have killed well over 2,000 people.
Joker has a variety of weapons at his disposal. For example, the flower he wears in his lapel sprays acid. The Joker has a gun which at first shoots a flag saying "BANG!", but then, with another pull of the trigger, the flag fires and impales its target. His most recurring gadget is a high-voltage hand-buzzer, which he uses to electrocute his victims with a handshake. Sometimes he commits crimes just for the fun of it, while on other occasions, it is part of a grand scheme; Batman has been noted to say that the Joker's plans make sense to him alone. His capricious nature, coupled with his violent streak, makes him the one villain that the other super-villains fear.