She won a gymnastic scholarship but still had that burning desire to earn a degree in psychology. Harleen never really earned great grades in that field, even though she tried her hardest. Crushed by her low markings, she seduced her professor into changing her grade to a perfect score, which was the highest grade of that year's students.

Harleen chose to intern at Gotham City's Arkham Asylum for the criminally insane because she dreamed of crafting a tell-all book about criminal mystique. There she met Dr. Joan Leland, who made Harleen feel comfortable in her new environment. She taught her everything she would need to know about the building, the doctors, and more importantly, the inmates. Only one inmate had Harleen totally fixated: the Joker, Gotham's Clown Prince of Crime.

When returning back to her office after a long day at work, she found a rose on her desk addressed to her from the asylum's very own cackling menace. Shocked, Harleen approached him, asking how he brought the rose into her office. Sporting a grin, he told her that he put it there and he loved her name. It reminded him of Harlequin, the classic French clown character of Commedia dell'Arte.

She pleaded with Dr. Leland for over three months to set up a session with the Joker. Once she succeeded in her begging, she studied all his tricks and gimmicks and felt she had read all the research that was needed to analyze him. Harleen entered the room, ready to hear anything, until the Ace of Knaves hit her with something that would throw her off course. He informed her of his abusive father and also torments from Batman. Harley had never realized all the torture that this poor soul had to endure.

As the young Dr. Quinzel checked up on him a few more times, she noticed that she had fallen in love with the Joker. When the Caliph of Clowns escaped from Arkham, Harleen went insane with worry. She read up on all his capers in the local Gotham newspaper, until he had been brought back to the asylum beaten to a pulp. Harleen's mind snapped in two. Any sanity that was left had been destroyed and she ran out of the asylum frantically screaming and crying.

She rushed to a nearby costume shop and stole various supplies to make herself over to her own persona to help her the Joker escape from Arkham once again, so they could live happily together. She remade herself into Harley Quinn, the Joker's obsessed moll and playmate. Harleen Quinzel was no more and Harley Quinn had taken her place.

Harley Quinn remains the Joker's special pet and self-appointed girlfriend. She does everything in her power to protect her boss, who she believes loves her. As a trademark, the Mistress of Mayhem and Mirth recites ridiculously witty lines to impress her Joker.