Harleen Quinzel was born on July 20, 1990, to an unidentified mother and father. She graduated high school and pursued a college degree in psychology. Following her graduation, Harleen's theatrical thesis made a huge success that it led to her transfer into the job as a psychiatrist at Arkham, When the Joker was imprisoned there by the masked vigilante Batman she took a particular interest in him, regularly visiting and speaking with him. Harleen was gradually psychologically sabotaged and manipulated by the psychopath, eventually falling in love with him, and the Joker got her to do him a favor, which was giving him a machine gun. Shortly thereafter, Joker's goons broke into the asylum and successfully broke their boss out. Before leaving however, Joker sadistically subjected Harleen to shock therapy, but she took the pain.
Would you die for me?"
"No. That was too easy. Would you... Would you live for me?"
"Careful. Do not say this oath thoughtlessly. Desire becomes surrender. Surrender becomes... power
Joker & Harley
Harleen later chased the Joker down and managed to get him to stop his car. She nearly killed him with a gun, even shooting a man who approached them. The Joker took the gun and taunted her. Harleen wanted to come with him, and he took her to Ace chemicals While there, she was told to say the oath and was asked if she would die for him. She responded in the affirmative and allowed herself to fall into the vat of chemicals that created the Joker years back. The Joker considered leaving her to die, but promptly changed his mind and jumped in after her, pulling her up to the vat's surface. He then kissed her, and Harleen awakened with the same frontotemporal dementia that her lover received via exposure to the chemicals and oxygen deprivation as the Joker broke down into maniacal laugherThe King and Queen of Gotham.
From that point on the duo of psychopathic lovers would go on a number of crime sprees in Gotham City together, which would often result in the duo coming into conflict with Batman