Even among the colorful denizens of Gotham City, Harleen Quinzel is unique. First introduced in the iconic Batman: The Animated Series, she wasn’t meant to stick around. But Harley was quickly recognized as a firm favorite of both the creators and fans, soon becoming a permanent fixture and eventually securing a place on DC’s A-list. She was even named "the Fourth Pillar of DC" (alongside Batman, Superman and Wonder Woman) by none other than Jim Lee!

Created by Paul Dini and Bruce Timm, Harley first appeared as a female assistant for the Clown Prince of Crime in the 1992 episode "Joker's Favor." She was originally conceived because the story featured someone jumping out of a cake and in pre-production the team thought it would be odd for the Joker to do it himself. Ironically, though, after she was designed, it wound up being the Joker who jumped out of the baked goods anyhow with Harley acting as one of his most dedicated—and maniacal—gang members.

Yet, while she was originally meant to just be part of a simple throwaway gag, Harley soon became a key recurring player on Batman: The Animated Series, with Arleen Sorkin’s iconic voice acting and Timm's immediately recognizable design making her a vital addition to the visual landscape of the show. Harley is unique in many ways, not least because she made the jump from the screen to the page and ended up becoming one of the most popular comic book characters in history in the process. Her first DC comic book appearance came a year after she debuted in the popular cartoon, with The Batman Adventures #12 bringing Timm and Dini's character to the comic store racks.

It would be four years later that Harley began to infiltrate the pages of the more mainstream comics, including those of the Bat himself. The first would be 1997's Elseworlds title Batman: Thrillkiller, which reimagined Dick Grayson and Babs Gordon as counterculture kids in the '60s. It makes sense that Harley's comic book life is intrinsically connected to Batman, as she was introduced in what many see as the definitive version of Bruce's life. In fact, her first solo title even included the name of her favorite frenemy, Batman: Harley Quinn #1. The still-popular one-shot was written by Dini and introduced Harley to DC continuity proper. It also introduced Harley to one of the most important people in her comic book life outside of the Joker, Poison Ivy.

Much of Harley's life has been defined by the men in it, whether it's her toxic and destructive on-off relationship with the Joker or her ever-shifting role as antagonist and sometimes ally to Batman. But Dr. Pamela Isley would go on to become her friend, ally, occasional enemy and eventual lover. Harley still had a long way to go until the pair would become one of DC's premiere queer couples, though, and her next step was her own 2000 solo series which ran for 38 issues over two years and featured contributions from creators like Terry Dodson, Karl Kesel, A.J. Lieberman and Mike Huddleston.

The first Harley Quinn series focused on something that would be a recurring theme for the anti-hero...she started a badass gang! This idea of creating a found family was something that would prove to be a theme throughout Harley's comic book career, and the concept of a cool girl gang began to fit even better with the increasingly feminist and hopeful narrative of Harley as she moved ever forward. Here, however, it was a way for Harley to gain control in Gotham. Eventually, after dying and being resurrected, Harley would decide to check herself in to Arkham Asylum in an attempt to gain help for herself and her struggles.

Dr. Quinzel would continue to appear in sporadic issues throughout the DC Universe that built on her ever-strengthening relationship with Batman and her unravelling one with the Joker until she took a starring role in the superhero team-up title Gotham City Sirens. The groundbreaking series, again written by Dini, saw Harley, Ivy and Selina Kyle's Catwoman create an alliance to protect Gotham. At first, Gotham City Sirens was an exhilarating ride as the women protected the city and Harley planned to break into Arkham and kill her abusive ex, the Joker. Unfortunately, in a twist that was surprising, but also represented a step back for the character, Harley teamed up with the famed villain, killing most of the civilians in the facility.

During the New 52 relaunch, Harley was redesigned and established as a key member of the Suicide Squad. In the process, her permanent split from the Joker began to be seeded, but it wouldn't be until 2013's Harley Quinn series from Jimmy Palmiotti, Amanda Conner and Chad Hardin that her independence and freedom would truly be cemented. The acclaimed creative team—arguably the most defining team for the character after Dini and Timm—reassessed Harley as a solid antihero who would more often than not fight against injustice even if her methods were somewhat questionable. It also saw her completely disconnect from the Joker, finally putting an end to the horrific abuse that had long defined her.

It was during this radical reimagining that Harley and Ivy embarked on their romantic relationship. While the pair aren’t exclusive, their relationship still made a huge impact on fans who had long seen the potential for the pairing and the love between the two.

Over the years, Harley has played many roles and worn many faces, but she's always inhabited a unique space in the world of Batman as a villain who has reformed and become DC’s most popular antihero, even turning her talents to true heroism on occasion. With her second live action film and first solo animated series both set to debut over the next year, it's clear that Harley Quinn’s time as one of the most important women in Batman’s canon is far from over.

Rosie Knight writes about comics, movies and TV for DCComics.com and DCUniverse.com. Be sure to follow her on Twitter at @RosieMarx.