On August 3, 1993, a comic came out that would prove to mark a pretty important change for Batman's gallery of foes: Kelley Puckett and Mike Parobeck's Batman Adventures #12. The story within, "Batgirl: Day One" is notable for a lot of reasons --- not the least of which that it's one of the best issues of that original run --- but there's one reason in particular that it'll always be remembered, because that issue marked the first comic book appearance of Harley Quinn.

Originally created for Batman: The Animated Series, Harley would go on to become not just a fan favorite, but the kind of character who would take a tragic, engaging, and occasionally hilarious hook and eventually become one of the core characters of the DC Universe.

Today's an interesting anniversary in its own right, too. It's not technically her first appearance, since she made her debut a year earlier in September of 1992 when Paul Dini and Boyd Kirkland's "Joker's Favor" hit the small screen as one of the most memorable episodes of Batman: The Animated Series, introducing Harley as a throwback to the beautiful henchwomen of the '60s television show.

It's not her introduction to the main-line DC Universe, either --- that would come in 1999's Batman: Harley Quinn, where the No Man's Land crossover would provide her with a backdrop to make a grand entrance alongside her frequent partner in crime, Poison Ivy. But it did mark the transition from one medium to another, and cement Harley as a character who would stick around for quite a while.

It's easy to see why. After "Joker's Favor," she was a fan-favorite almost instantly, thanks to a combination of a beautiful, color-blocked design from Bruce Timm, and Arlene Sorkin's trademark voice (complete with the sing-song heavy accent that would give her the unmistakable "Mistah J"), and she only became more compelling once we found out more about her.

With stories like Paul Dini and Bruce Timm's Eisner award-winning "Mad Love," which gave her origin as one of Arkham's psychiatrists who fell in love with the Joker and turned to a life of crime, and animated series episodes like "Harlequinade" and "Harley's Holiday" (both written by Dini, the creator most prominently identified with Harley), she'd emerge as one of the most sympathetic figures in the Batman mythos.

She's the sort of character that we'd never really seen before in that context, and it made her work in a way that went far beyond just playing sidekick to a villain. She's a hopeless romantic, someone whose love for the Joker gives her a central tragedy that's so recognizable and heartbreaking that you can't help but sympathize with her, even while she's aligned with the most evil villain in Gotham City. She's part of Batman's world in a way that no one else is, and that makes her great.

And it's that same compelling tragedy that made her emerge as a star in her own right. It wasn't long after she made her first appearance in the comics that she was given an ongoing series of her own and a starring role in Gotham City Sirens, and when the DC Universe was rebooted for the New 52, she became the central figure of Suicide Squad (and the live-action film of the same name) before getting another series of her own. Through all that, though, she remained linked to the Joker, the anchor for the appeal of her heartbreaking tragedy, and to Poison Ivy, the relationship that would eventually find her becoming one of the company's most prominent, canonical LGBTQ characters.

And it all started over 20 years ago today, when Harley took the first step towards crossing over into a different medium, becoming one of the most successful new characters in decades, and cementing her place as one of comics' favorite characters.