Twenty-five years after her introduction on Batman: The Animated Series, Harley Quinn is a certified pop-culture icon, inspiring an avalanche of merchandise, headlining comic books and a live-action film (with more in development), and sharing the spotlight with the Dark Knight in the animated Batman and Harley Quinn.

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But what's fueled the character's meteoric rise? Veteran Batman voice actor Kevin Conroy has a theory.

"I think it has a lot to do with The Joker," he told HeyUGuys while promoting the release of Batman and Harley Quinn. "Before Harley Quinn, the only foil for The Joker was Batman. The Joker defines Batman and Batman defines The Joker. Mark [Hamill] and I have often approached the roles as the yin and yang of each other but I’m like the dark grounding for Joker, he had no one to really go crazy with and Harley Quinn gave him that."

"She got to be this crazy reflection of the Joker, so I think the better Mark was, the better Harley Quinn was. A lot of her popularity grows out of people’s love for The Joker. They love Mark as The Joker," Conroy continued. "She gets reflected glory from that character but Arleen Sorkin, Tara Strong and now Melissa [Rauch] then took that and made it this crazy, gum-chewing, wise-cracking New Jersey broad who’s just crazy, crazy evil and so much fun."

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"So, I think the birth of it was Joker and his popularity, but then each of those actresses just took it and ran with it," he added. "There’s also a big new female audience in animation and they’re looking for more interesting characters for them and, you know, Wonder Woman is a wonderful superhero but she’s not very multidimensional, whereas Harley Quinn is just nuts and just has so many different qualities that it’s fun for the female audience." he concluded.

Margot Robbie is expected to reprise her role of Harley Quinn in the sequel to 2016's Suicide Squad, as well as in the planned spinoffs Gotham City Sirens and Joker & Harley Quinn. Batman and Harley Quinn is available now on digital download, Blu-ray and DVD.