The Joker is the super criminal and number one arch-Nemesis of Batman.
A heroic alternative reality version of the Joker called The Red Hood appears and is voiced by Jeff Bennett. He is shown to be an extremely capable fighter, able to hold his own against multiple members of the Injustice Syndicate. In addition, he wields projectile weapons shaped like spades (a reference to his alternate universe counterpart's playing card motif). Like the Joker, this Red Hood was disfigured after falling into a chemical vat at the Ace Chemical Plant. In his case, he was already a superhero and was actively dropped in by Owlman. His sanity is said to have been "Bent, but not broken." Red Hood then tries to rally his world's heroes (alternate versions of the villains from the "normal" universe) against the Injustice Syndicate, but they are defeated. Red Hood escapes and tries to use a device to recruit help from an alternate Earth (Batman's universe), but he is captured by the Syndicate. After Batman is attacked by his alternate-reality doppelganger, Owlman (sent to Batman's universe on a reconnaissance mission), he journeys to Red Hood's dimension. During scenes in this episode, the Red Hood's face is shown, but in shadow, showing a bit of green hair and a wide grin that clearly resembles the Joker. After the heroes are freed and the villains defeated, Red Hood thanks Batman and hopes his counterpart can return the favor. Sure enough, in the next episode "Game Over for Owlman!", Batman is forced to team-up with the Joker in order to defeat Owlman, who has impersonated Batman and ruined his reputation in his absence. When the Joker asks Batman what made him takes his chances working with him, Batman briefly has a flashback of his last moment with Red Hood and replies "Something a friend said".
The Joker himself made his debut on the show in the episode "Game Over for Owlman!" (a continuation of "Deep Cover for Batman!") also voiced by Jeff Bennett. His appearance and personality is very similar to the Silver Age version, as drawn by Dick Sprang. With the police and some of his superhero friends after him, Batman has no option but to team up with Joker to stop Owlman's crime spree especially when Owlman was upstaging Joker. During that time, the Joker briefly becomes a hero, but ultimately goes back to his evil roots because being a good guy was "just not him".
Joker makes an appearance in "Legends of the Dark Mite!" along with the Penguin and other Batman villains.
Joker appears in the teaser to "Hail the Tornado Tyrant!" when he is being tailed by Batman and Green Arrow during a series of robberies. He is captured after his car's many means of propulsion are destroyed by the combined efforts of the heroes.
Joker also appears in the episode "Death Race to Oblivion!" as one of Mongul's racers. He creates tough obstacles for Batman and the others until after he is out of the race. He occasionally provides a play-by-play of the race while doing so. He is transported to a cell with the other losing villains and later put in a green cell created by Guy Gardner's power ring.
Joker appears again among other villains in a bidding for a supersonic weapon held by arms dealer Joe Chill in the episode "Chill of the Night!"
He later appears in "Emperor Joker!" as the primary antagonist along with his love, Harley Quinn and again in the episode "The Knights of Tomorrow!".
In the Scooby-Doo crossover created by Bat-Mite, Joker was voiced by Corey Burton.
In typical Joker style, he takes over an episode, which is called "Joker: The Vile and the Villainous!" where the roles of protagonist and antagonist are reversed. Even the theme song is a "Jokerized" version, with several of his fellow criminals shown.
He reappears in "Night of the Batmen!", where he plans to make everyone on Earth insane just like him. He also captures Green Arrow, Plastic Man, Aquaman, and Captain Marvel: all were substituting for Batman, who was critically injured while fighting Kanjar Ro. Fortunately, Batman makes a special exosuit to free his partners and foil Joker. In one last attempt to kill the Dark Knight, Joker tries to electrocute him with his Joy-Buzzer, but Batman anticipated this and counterattacks by shocking Joker himself.
He also appears in both the opening and the main plot of "Triumvirate of Terror". In the opening, he was seen playing baseball with the Legion of Doom against the Justice League International. In the main plot, Joker tries to kill Batman and Vicki Vale by launching a giant exploding pie onto them. But, as a predictable result, Batman escaped, freed Vicki, and dodged the pie before it hit. However, the Joker managed to make his escape. He later met up with Lex Luthor and the Cheetah, both who were having trouble dealing with their enemies Superman and Wonder Woman, respectively. They agree to switch enemies to destroy them. As such, Joker went after Wonder Woman. However, before he could kill her with a giant hammer, Luthor teleported the heroes and villains to their hideout, where they were going to kill them all on a worldwide broadcast. Fortunately, just as the villains switched heroes to fight, the heroes did the same, triple-teaming on them to defeat them.
He also appears in the opening for "Powerless", where he has assembled a group called the Jokers of All Nations to fight the Batmen of All Nations. This team is made up of an Inuit, a hockey player, a Scotsman, a Cossak, and a sumo wrestler.
Joker also appears in the opening for "Crisis: 22,300 Miles Above Earth" where he was hosting a celebrity roast to literally roast Batman. He has also invited Clock King, Gorilla Grodd, Penguin, Two-Face, Poison Ivy, Mr. Freeze, Kite Man, Gentleman Ghost, Black Manta, Riddler, and Solomon Grundy to the roast. The Joker has even forced Jeff Ross to roast Batman, but it turns out that Ross was a distraction for Batman to escape the roasting stick. When Joker tried to escape, Jeff got in his way and knocked him out.
His final appearance was in "Mitefall" as one of the guests at the series cancellation party set up by Ambush Bug. He was mainly seen alongside Harley Quinn.