The Joker is a deranged clown-themed psychopath who seeks to cause chaos at every opportunity, especially that which torments his arch-enemy Batman. He was eventually defeated by the Green Lantern John Stewart and imprisoned in Supermax 282, but escaped during a breakout caused by Strika several years later. His current whereabouts and activities are unknown.
Schemes on Earth
In 150,081 CMT, the Joker used combustible lemons to burn down several orphanages and get the attention of Batman, who he hoped to lure into a trap. As planned, Batman came to confront the Joker: when he did so, the Joker and his girlfriend Harley Quinn ambushed and attacked him, but they were both overpowered and taken to the Hall of Justice. Batman attempted to interrogate the Joker, but lost his temper and used Quinn as a makeshift club to beat the Joker with while yelling at him to stop burning down orphanages with oranges. Cave Johnson interrupted the brawl to correct Batman in that the Joker used combustible lemons, not oranges, to burn the orphanages.
In 150,082 CMT, the Joker hatched a plan to release a chemical into Earth's oceans that would cause all of Earth's fish to bear his likeness. To further this goal, he went to Atlantis and gained access to the underwater city by persuading its queen, Mera, that he was an ally of her husband, Aquaman. The Joker then attacked her, chained her up, and did the same to her husband when he returned. The Joker gloated to them both about his plans, but he was stopped by the sudden arrival of the Green Lantern John Stewart, who incapacitated the Joker with a construct resembling a gigantic fist.
The Mazkertis Paradox
In 150,084 CMT, President Trump considered that the violent and chaotic nature of the Joker could be an asset if it was used against his enemies. The fact that Mazkertis had embarrassed Trump earlier that year by rejecting an offer of his in front of the Kindred of Dawn instilled a desire for revenge in Trump, so he decided that he would eventually attempt to free the Joker from Supermax 282 and send him to kill Mazkertis with some other "aces" that were trapped in the prison: Seeldier and Agent 47. However, Trump first had to attend an important assembly of the Galactic Government, so he decided to wait until after the assembly to work on a plan to free the prisoners.
To Trump's surprise, when the assembly of the Galactic Government rolled around, it was interrupted by four characters who also wanted to see Mazkertis defeated: Captain Eurobeat, Strika, Niflheim, and Levi. The four intruders requested the attending delegates' help in locating Mazkertis and putting a stop to his reign of terror, but the chairman of the meeting and most of the diplomats were uninterested, so Trump floated down on his podium to meet them before they could leave empty-handed. He made them an offer: if they rescued three assassins from Supermax 282 (the Joker included), they could use them to kill Mazkertis, and would secure the permanent allegiance of Earth in their activities by doing so. The aliens agreed to Trump's proposition, so he gave them the mugshots and identification numbers of the Joker, Seeldier, and Agent 47, and directed them towards Supermax 282.
When the allies arrived on Supermax 282, they discovered that the only way to free their desired prisoners was by blowing a power box that would cut power to all the cells' locking mechanisms, freeing all the prisoners at once. The allies were able to intercept and secure the allegiances of Agent 47 and Seeldier before they could be lost in the crowds running for the exit, but unfortunately, they lost sight of the Joker in the ensuing chaos. Captain Eurobeat showed Agent 47 the picture of the Joker and asked him if he had seen him; 47 responded by telling Eurobeat that the Joker wouldn't have been much use to them anyway, because, although the Joker had talent, he was "unreliable and absolutely insane", and that it "probably won't be long before he's caught again and put back where he belongs".