Jerome first made contact with the Joker when he and some friends tried to summon the Smiling Man by telling sick jokes in front of a mirror. Jerome won, and after this was haunted by seeing a grinning visage whenever he looked in the mirror. This eventually drove him insane leading to him embracing the Joker. He then put on a smile mask that totally bonded him with the Joker.
The Madman who Laughs
Joker's Five-Way Revenge
After his last caper, Joker was captured and sent to a Mental Hospital for the Criminally Insane. Joker's mental condition worsened and when he learned that one of his thugs had double-crossed him, Joker broke out of the hospital and went on a murdering rampage, killing his former henchmen using his trademark Joker Venom, among other methods. With this new criminal mindset, Joker was able to knock Batman unconscious and had the chance to kill him once and for all, but he refused to do so and in the end, it allowed Batman to capture the Joker and bring him back to the authorities.
The Laughing Fish
Joker eventually escaped and set his next plan in motion and he used a chemical across the shores of Gotham that contaminated the waters, transforming the fish to grotesque creatures with Joker's face on them. Joker's goal was to secure trademark for the “Joker Fish”, but when he was denied, because of the law, Joker started a killing spree against public servants, announcing their deaths and killing them with his trademark Joker Venom. Joker managed to claim two victims thanks to his careful planning, but during his third murder attempt, he was stopped by Batman. Joker tried to escape, but Batman chased him through the rooftops of Gotham until they reached a construction site, where Joker was shocked by lightning and fell to the river below. Joker didn't come up after that last blow, but this was not his demise.
Redlodge: Year One
Joker was one of the many villains recruited by Double to disrupt the wedding between Batman and Catwoman. He was not apprehended.
Joker's First Laugh
Joker returned with the power of the Q to menace Gotham City. He then replicated some of his greatest crimes, targeting Leo Redlodge, whom he hoped to make into a new Joker. He captured Redlodge dressed him like the Red Hood and pushed him into the acid vat. Redlodge was rescued by Batman and Joker's power was stolen by the Q. He was then beaten up by Redlodge and hauled back to Arkham.
Joker was one of the many villains recruited by the Master Griefer to defeat Leo Redlodge. He was the one who cut out Redlodge's eyes. He teamed up with the Scarecrow after that, but was defeated by Leo Redlodge Jr. and some Gotham Academy students.
Joker teamed up with Forrest Blackwell to learn the secrets of Disaster. Joker was tasked with starting a nuclear war between the U.S. and North Korea. This failed and he was apprehended. He then broke out of prison and tried to kill Donald Trump (actually Blackwell in disguise). The two trapped Redlodge and tried to drive him insane. Redlodge managed to recover from their torture long enough to escape. He then blew up the island with Blackwell laughing and Joker trying to convince Redlodge to spare him.
Joker teamed up with Oilhand to trap Batman in a funhouse. Oilhand tried his best to kill Batman and his friends but failed. Oilhand then shot Talia. This shot did not kill her, but Joker then did. Joker then remarked "She wasn't right for you. I'm the only one you can ever love. You need me Batman, and I'll kill everyone you think you love to prove it to you" Batman then flew into a rage and beat the Joker to death saying "this is how I need you Joker. Dead!" He then took Joker's mangled body to Arkham which he paraded around saying to the other criminals, "Batman kills now." Joker was buried in the Arkham graveyard.
Batman: The Faith of Smiles
Joker was dug up and resurrected by a Vecna worshipping cardinal. This cardinal seemed to be the likely candidate for the next pope and asked Joker to kill the pope. The Joker did and then killed the cardinal. He then impersonated him. Cardinal Joker then led a seance with God (since "God" is subjective, they summoned Vecna instead). Vecna stated that it was the churches's duty to kill everybody. They then elected pope Joker. He was of course disguised from the world as the cardinal he killed. He then struck a deal with Tymaret that he would bring him Damian Wayne. His plan was to go on tour in Gotham and state that a killer called Smiler (actually himself) wanted to kill him. This would force Batman into "protecting him" where he could grab Damian Wayne.
After doing this, Joker started to torture Damian stating he would kill him in exactly the same way he killed his mother. Joker got greedy however, and tried to bargain with Batman saying he would give him Damian if he and Redlodge converted to Jokerism. They both agreed. However, before he could get the ceremony ready, Damian broke loose and beat him into a coma. Bruce stopped Damian from killing him saying, "he deserves to die, but you don't deserve it on your conscience." He was then placed in stasis at Arkham, although the underworld was informed of his "death".
The Joker Reforms
Soon, a dangerous even more unpredictable maniac named the clown appeared, causing more panic than the Joker ever had. The Clown released the Lost Inmate, which made the government declare Gotham a No Man's Land. However, just as Clown and the Lost Inmate were going to do some unspecified terrible act, Joker appeared and he and clown fought. He defeated Clown, and killed him, saying "its not funny if you kill everyone at once."
At this point, Joker was informed of the existance of his long lost son Jack, this made that "One bad day" not seem so bad anymore. Joker than suddenly became sane, and was soon released from Arkham, and allowed custody of his son.
Joker took a job as a security chief of Waynetech. He helped protect Gotham from Tymaret, but his son was killed by the Heretic. Joker then became insane again, Blackwell resurrected Jack, but Joker decided to remain insane since it was easier that way. He then temporarily joined with Tymaret but backstabbed him and joined Mr. Shadow's Secret Six. He destroyed the Resurrection Mask by slicing it in half with a razor joker card. After the death war, he remains at large.