Over the years, Harley Quinn has been through many different changes. Following her debut in Batman: The Animated Series, Harley made her way into the comics, before rising in popularity in the greater DC Universe. One of the biggest factor's contributing to Harley's popularity has to be joining the Suicide Squad during the New 52. Harley was recruited onto Task Force X in 2011's Suicide Squad #1, by Adam Glass, Federico Dallocchio, Scott Hanna, Ransom Getty, Val Staples and Jared K. Fletcher. In her New 52 debut, Harley was arrested by Black Canary while in the midst of a killing spree. The Joker had gone missing, after having his face cut off in 2011's Detective Comics #1, by Tony S. Daniel, Ryan Winn, Tomeu Morey and Jared K. Fletcher.
Failing to get the Joker's attention, Harley was taken to Belle Reve, where Amanda Waller abducted her to join the Suicide Squad. Of course, Waller only recruited Harley because she knew the Joker would return, using Harley against the villain when he did. Even so, this was a major change for both Harley and the Suicide Squad, since Harley had never been associated with the team before the New 52. Now, however, Harley had a place on a major DC team, taking her out of Gotham.
Previously, Harley had made moves to get out of the Joker's shadow, by teaming up with Poison Ivy and Catwoman, and through her solo adventures, but she'd never fully escaped Batman's world. On the Suicide Squad, however, Harley found a new outlet for her madness, working with some of DC's most notorious super-villains in the process. Over time, the Suicide Squad helped Harley forge her own path, away from both Batman and the Joker. The Suicide Squad furthered Harley's path to redemption, which she'd been traveling for years before. Now that she was out of Gotham, Harley could break the cycle in which she'd been trapped, flipping back and forth between villain and anti-hero.
By 2013's Suicide Squad #16, by Adam Glass, Henrik Jonsson, Sandu Florea, Matt Yackey and Jared K. Fletcher, Harley finally decided that she was finished with the Joker. Most noticeably, when Harley joined the Suicide Squad, she took on a whole new appearance. Harley abandoned her old jester outfit, which she'd worn since her introduction to comics. In the New 52, Harley wore an outfit inspired by the acclaimed Batman: Arkham series of video games. Since then, Harley has gone through a few costume changes, but they've all been based on this more modern look. Of course, the Suicide Squad didn't just change Harley. By joining the team, Harley drastically altered the Suicide Squad.
The team had been going for decades before Harley joined, yet, ever since Harley became part of the Suicide Squad, she's been a key member of the group. In each successive volume of the Suicide Squad comic since the New 52, Harley has been featured. In other media, from animation to film, Harley has also been heavily featured on the Suicide Squad. Harley was also so popular in 2016's Suicide Squad that she starred in her own film, 2020's Birds of Prey. Clearly, joining the Suicide Squad has increased Harley's popularity in the DC Universe overall. After breaking out on the Suicide Squad, Harley has been featured in several ongoing solo series of her own.
In recent Batman comics, Harley has also become an unofficial member of the Bat-Family as well, cementing both her character growth and rise in popularity. Outside of the comics, Harley has also gained her own animated series, showing how recent years have been especially kind to the character, thanks to the Suicide Squad. Harley's early popularity which led her to join the Suicide Squad in the first place. From there, the cycle just continued, as Harley received more attention by joining the Suicide Squad. By giving Harley Quinn her own team, and a place outside of Gotham City, the Suicide Squad made an already iconic character into a true pillar of the DC Universe.