Since her introduction in Batman: The Animated Series, Harley Quinn has become a fan-favorite villain among comic readers, moviegoers and Halloween costume enthusiasts alike. While she's a relatively new character in comparison to the rest of Batman's notorious rouges gallery, she has still had her origin retold a number of times, one of which was during the cataclysmic "No Man's Land" event that brought her into the main DC Universe.
While she first appeared in the Batman: The Animated Series episode "Joker's Favor" written by Boyd Kirkland and Paul Dini in 1992, her origin wasn't revealed until the one-shot comic The Batman Adventures: Mad Love by Dini and Bruce Timm the next year. This was adapted into an episode of the same name in 1999's The New Batman Adventures, which had a nearly identical story.
In this story, Doctor Harleen Quinzel was working in Arkham Asylum as a psychiatrist and was especially drawn to those with criminal insanity. After working with the Joker, she felt an attraction between them and stopped seeing him as a dangerous criminal, but as a psychologically vulnerable man, and eventually fell in love with him. After he'd escaped one night, Batman returned him bloodied and bruised, which devastated Harleen. That night, she went into town, purchased a black and red clown costume and released the Joker, joining him on his future crimes as Harley Quinn.
In many ways, this is similar to her origin in Batman: Harley Quinn by Paul Dini, Yvel Guichet and Aaron Sowd, which was a tie-in to No Man's Land, released later in 1999. As in Mad Love, Harleen Quinzel starts as a psychologist working with the Joker and slowly falls in love with him. Rather than escaping with him spectacularly, she repeatedly helps him out of the Asylum before she is caught by Jeremiah Arkham, the warden, who punishes her by putting her in solitary confinement. When the asylum is wrecked by an earthquake, she escapes into town.
She then loots a costume store, dons her now-iconic outfit and manages to find the Joker in the middle of a fight with Penguins henchmen. Joker recognizes her, they retreat to a hideout she decorated for them, and together they slowly begin to take control of the surrounding areas. After a few encounters with Batman, Joker tricks Harley into going inside a rocket, telling her he couldn't stand feeling the same as she did and had to dispose of her. She managed to take control of the rocket, crashing it into Robinson's Park, where she is discovered by Poison Ivy.
Feeling pity, Ivy gives her a drink that gives her immunity to toxins, as well as increasing her agility and strength, which is similar to an injection she gives her in the Batman: The Animated Series episode "Harley and Ivy." The two of them manage to contact Batman and tell him of Joker's plan to kill hundreds of people. After tracking him down, Harley attempts to kill both Batman and the Joker, switching attacks between the two and getting the best of both of them. Just before Harley unleashes the killing blow, Joker apologizes to her, and she instantly accepts the apology. The two of them escape and leave Batman at the hideout to be blown up.
Since then, a number of changes have been made to Harley's origin. In current continuity, the beginning of Harleen Quinzel's transformation has remained her infatuation with the Joker, but her insanity is instead a result of her taking a dip in the same vat of chemicals that were believed to have bleached the Joker's skin. This is the case in the New 52 comics, the current Harley Quinn animated series, and the 2016 film Suicide Squad, although in the latter she jumps in by choice, whereas the other two iterations see her pushed by the Clown Prince of Crime. While these may have caused more permanent cosmetic changes, Batman: Harley Quinn gave us an origin that showed her not only as a woman in love with a twisted villain but a formidable villain who could go toe-to-toe with both the Joker and Batman himself.