The Joker is the Clown Prince of Crime of Gotham City.
The Birth of Harley Quinn
At Arkham Asylum, the Joker developed an interest in his psychiatrist, Dr. Harleen Quinzel (of which later on she did the same). Harleen brought her patient a teddy bear and Joker asked her for a machine gun. Escaping, Joker ordered his men to tie Harleen to a bed for him to torture her, in the hopes of driving her insane. Later, Joker brought Quinzel to Ace Chemicals and threw her to a vat. Joker thought of leaving Harleen to drown, but had a change of mind, and saved her. He then kissed her and she woke up and they both laughed.
Out of all of Batman's enemies, the Joker was the one who made him suffer the most. Little was known about that event. What was known, however, was that the Joker and his girlfriend, Harley Quinn, kidnapped Robin, Batman's ward and adopted son, tortured, and killed him. For that, Batman broke Joker's front teeth and sent him to Arkham, where he got a "DAMAGED" tattoo on his forehead.
Business with Monster T
In an strip club, the Joker met with local gangster Monster T, who had hopes of making business with the Clown Prince of Crime. During their chat, Monster T looked at Harley, who acted like a stripper, much to Joker's dismay. The Joker called Harley and ordered her to have sex with Monster T, which caused the gangster to become afraid and keep trying to say to Joker that Harley was his girl. Joker eventually shot Monster T in the head. Harley and Joker left the strip club, but Batman appeared. Joker managed to escape, and left Harley to be arrested by the Dark Knight.
Comatose without Harley
Harley was arrested and sent in Belle Reve and was soon to be a member of Task Force X, otherwise know as the Suicide Squad. Meanwhile, the Joker was in a room full of knives, guns, and other bizarre things, feeling sad without his girlfriend and partner in crime, when his henchmen Jonny Frost arrived and said that Harley and other people were missing after going to a "swamp in Louisiana". The Joker said to Jonny to get the car, because they were going for a ride.
Assault on Wayne Enterprises
The Joker and his men assaulted a Wayne Enterprises factory, where the bombs for Task Force X were produced. Joker showed Dr. Van Crisis, the one behind the explosives, his wife held hostage, which lead the doctor to deactivate Harley's bomb.
After they stole a helicopter, the Joker and his gang began an operation to rescue Harley. Arriving at the rooftop, where the Suicide Squad and Amanda Walller were awaiting Waller's rescue, the Joker and his men opened fire at them with Harley climbing a rope to the chopper. The couple hugged and kissed, but their happiness was short lived as Waller ordered her men to shoot down the helicopter. Joker threw Harley off the air vehicle, but apparently fell victim to the crash.
Back with Harley
After he somehow survived the crash, the Joker and his men broke into Belle Reve and rescued Harley. The couple then headed back to Gotham. However, they ultimately broke up for hitherto undisclosed reasons.