The Joker was a psychopathic mass murderer who was confined to a psychiatric facility after the retirement of the Batman.
All-Star Batman and Robin the Boy Wonder
Joker had hired the man who murdered the parents of Dick Grayson and planned something large for the Dark Knight. His start had been an attempt at poisoning the Gotham reservoir, and he soon became one of the worst enemies of Batman. He was known to kill for sick pleasure, often indulging in tawdry acts with women beforehand.
One such woman was Donna Gugina, an upstanding attorney whom he spent a night with and then brutally attacked, crushing her larnyx.
After having his henchwoman Bruno clean up the mess, he contacted Selina Kyle, the Catwoman. Whatever transpired left her beaten, as she was later found by Batman in such a state.
The Dark Knight Returns
After Batman's retirement, Joker was driven into a state of catatonia for ten years after the disappearance of his old foe. Largely because of his catatonia, Joker was considered the favorite patient of Arkham Asylum psychiatrist Dr. Bartholomew Wolper. However, once Batman took to the streets again, Joker re-emerged from his catatonia and sought to battle the hero once more. Abner found out, as he was the most faithful of Joker's old hirelings apart from the still-active Bruno. After telling Joker that Two-Face had commissioned his help in making bombs for a ransom, Joker had him sabotage the devices so that they would prematurely explode.
He manipulated his psychiatrist Dr. Bartholomew Wolper into thinking that he had been cured from his psychopathy, while putting Abner to use, having him fashion some deadly toys for a coming killing spree. Wolper then managed to get him on a talk show to talk about his new self, but was killed along with everyone in the same room. Everyone except for the Joker, that is. Batman had tried to intervene and prevent what he knew would take place, but because of the untimely intervention of the bumbling police, he failed to stop the massacre or do anything but risk his own life.
After his escape, the Joker appeared at Selina Kyle's escort service's headquarters and from there used a special brand of lipstick previously given to him by Abner, taking control of Selina's mind and that of one of her girls. The girl transmitted Joker's influence into a congressman by a kiss, resulting in his ranting for a strike on the Soviets and subsequent death from a fall. After dressing Selina up as Wonder Woman, beating her in the act, the Joker left her tied up and sobbing. Batman and Robin soon arrived and rescued Selina, the later finding a clue as to where the next crime would take place in some stray cotton candy left on the floor.
Joker then made his way to an amusement park with Abner in tow where they plotted out eventual mass murder utilizing a collection of explosive flying dolls filled with gas, until Batman arrived and duelled his old foe, promising to end his life. Robin herself was shocked by what the Joker and his sidekick had done, soon coming into conflict with Abner while Batman took on his old foe. In the battle, Joker proved to be exceptionally fast as he managed to shoot Batman in the stomach and stab him there several times, but Batman flung a Batarang into Joker's eye and broke his neck, paralyzing, but not killing him. Batman found himself unable to kill the Joker, to which Joker, out of disappointment, twisted his own broken neck while cackling cruelly in order to frame Batman for murder. Even with his own death, there was a sense of unfinished business between the two.
In Other Media
The Joker only appears in the very end of the first part, where he breaks out of his catatonia upon hearing the news reports of the Batman's return, only saying in a sinister tone "Batman... darling." In addition, he was mentioned earlier in a conversation between Commissioner Gordon and then-retired Bruce Wayne while discussing their retirements (or in Gordon's case, upcoming retirement), citing that Batman's absence resulted in Joker being catatonic and hoping he stays that way. Because of Two-Face's actions at the Gotham Life Towers being moved to before the Mutant Leader was defeated, the Joker was not responsible for Two-Face's actions.
He plays a more active role as the main antagonist of Part 2. Like in the comics, he tricks Dr. Wolper into letting him appear on a talk show after falsely presenting himself as having been cured, a move that was considered controversial as soon as it was announced. He eventually murders Wolper by slitting his neck with a novelty mug (referring to a similar action the Joker took in Batman: Under the Red Hood against one of Black Mask's henchmen), and also arranged for his robotic dolls, Bobby and Mary, to arrive and gas the audience and talk show host with Joker venom. He also breaks into Selena Kyle's apartment and uses a special lipstick to brainwash her and one of her escorts to manipulate a senator into starting a nuclear war with the USSR. He then murders the senator and beats Selena Kyle into submission after dressing her up as Wonder Woman before departing for the carnival. He then sells poisoned cotton candy to boy scouts (their deaths are implied but never actually shown). Upon noticing Batman and the new Robin's arrival on the Bat glider, he notifies Bobby and Mary about their arrival, and tells Bobby to attack them. However, after Bobby was destroyed without actually hurting Batman or Robin, he has Abner and Mary pursue Robin while he personally deals with Batman. He then gets into a shootout with Batman, and eventually holds Robin's friend hostage, with Batman throwing three Batarangs at him to force him to let go. Joker, not anticipating that Batman would use that kind of attack, was shocked, and lost his right eye in the process. He then runs into the house of mirrors and takes a boy hostage after murdering his father and shot at Batman. However, Batman used the reflections to his advantage to confuse the Joker enough to ambush him and free the boy. Despite this, the Joker managed to shoot Batman in the chest, weakening him, although he ran out of bullets before he could finish him off, although he managed to escape due to pinning Batman down with one of the mirror walls. He then "surrendered" to a security cop, only to murder him with a knife in his sleeve, and stole his gun. He then shot at Batman and destroyed part of a bench. He then slaughtered several attendees near and in the Tunnel of Love, although Batman eventually caught up with him and beat him down. Ultimately, the Joker had his neck snapped by Batman in self-defense, although he was still satisfied that he got Batman to "lose control" and proceeded to frame him for murdering him by finishing off the neck snap himself, although not before cruelly telling Batman that he'll see him in hell. Joker's body was later burned in an explosion, although his laughter penetrated.
He was voiced by Michael Emerson, who based Joker's voice on comedian Paul Lynde.