Dr. Harleen Quinzell was a young psychiatrist of questionable skill and ethics who had talked herself into an internship at Gotham City's notorious Arkham Asylum. She had high hopes of taking what she witnessed there and turning it into an exploitative bestseller on its famous inmates.
Unfortunately, she ended up falling madly in love with the Joker, who ran a number of mind games on her during her attempts at therapy with him. Her own sanity took a serious beating, she became his secret accomplice, aiding him in his many escapes from Arkham for some time before she was eventually caught and committed to the asylum herself.
After several long months in solitude, Harley was surprised one day to find the door of her cell unlocked. Venturing out, she discovered a shattered and abandoned Arkham, ruined by the recent slew of disasters plaguing Gotham. She made her way into the city in search of the Joker, adapting her current disguise and demeanor in hopes of winning his favor.
She succeeded with flying colors after saving his life in an encounter with The Penguin The Joker took a liking to Harley, exploiting her for labor and company, and with Harley loving every minute. She deluded herself that her love was returned.
But the delusion was shattered when the Joker, now settled into a new power base, decided she was no longer useful. He packed her into rocket and shot her halfway across the city to crash and burn.
Harley managed to bring the rocket down mostly safely and to survive under the wreckage. She was found there by Poison Ivy, who took her home out of curiosity, and listened to her story. Ivy nursed her back to health out of sympathy and in the hopes that she would be able to use Harley against Batman. %r Harley mixed it up with both Batman and the Joker, and came close to doing them both in, in a vengeful rage before falling once again for the Joker's alleged charms, and rejoining him. The two remain at large in the ruins of Gotham, no doubt plotting more chaos.