By: Henry Hanks, Contributor @hankstv / email@example.com
When the Joker introduced a sidekick named Harley Quinn in an early episode of "Batman: The Animated Series," no one could have predicted it would be the start of a phenomenon.
That moment 25 years ago would eventually lead to Harley becoming one of the most popular characters in the DC universe, rivaling Batman himself.
As such, this year's annual Batman Day (this Saturday) is being taken over by Harley.
Little was known about Harley's background until the Eisner Award-winning 1994 graphic novel "Mad Love," (by "Animated Series" creators Paul Dini and Bruce Timm) which explained how psychologist Harleen Quinzel fell for the Joker and joined him in a life of crime.
However, Harley first appeared in comics in "Batman Adventures" #12, which has since become a highly sought-after issue.
After several more appearances in the "Adventures" comics, Harley joined the proper DC universe in 1997 in her own one-shot comic, and got an ongoing series in 2001. After that ended in 2003, she continued to appear sporadically in DC Comics, and had another mini-series "Harley and Ivy" (with her BFF, Poison Ivy) in 2007.
Catwoman joined the pair in the series "Gotham City Sirens" in 2009 (which is set to become a movie in the near future).
Harley's look had been reinvented a few times by this point, and was once again for her ongoing series as part of the New 52 in 2013. The husband and wife team of Amanda Conner and Jimmy Palmiotti have worked on Harley's series ever since, and again when it was relaunched with DC Rebirth last year.
This series has been wildly popular for the last four years, and of course, she broke into mainstream pop culture with last year's "Suicide Squad," starring Margot Robbie as Harley (the comic book Harley has been revamped to resemble Robbie's version).
Anyone who attends fan conventions knows how popular Harley Quinn cosplay has been for many years, and now that she's a household name, there's no telling how big she will get.
Very impressive for someone who was just a mere sidekick 25 years ago.