The fighting prowess of Harley Quinn cannot be understated, and during a short period where she took up the identity of Batgirl, she even took down trained assassin Cassandra Cain, aka the actual successor to Barbara Gordon. Harley is known for taking on heroes, villains and monsters in fights that seemingly put her at a disadvantage. But a misadventure in Gotham found Harley in a match-up with the deadliest Batgirl that nearly ended with her usurping the role.
Harley Quinn was created by Paul Dini and Bruce Timm for the 1992 cartoon Batman: The Animated Series, first appearing in the episode "Joker's Favor." She quickly became a fan-favorite and recurring character in multiple DC Animated Universe shows. Her popularity led to comic appearances such as the Elseworlds title Batman: Thrillkiller and animated-canon series The Batman Adventures, before making her mainstream appearance with Batman: Harley Quinn #1 in 1999. A year later, Harley would get her first self-titled comic that would see her come face-to-face with a number of DC icons, including Cassandra Cain.
Harley Quinn by Karl Kesel and Terry Dodson finally gave the spotlight to Harley and was the first attempt at making Harley more than just the Joker's sidekick. In a big step towards her personal growth, Harley realizes for the first time that her puddin' doesn't value her as much as she though. The epiphany leads Harley on a journey of self discovery but also puts her in the crosshairs of a gangster that puts a hit out on her. To stay ahead of the baddies coming to collect the price on her head, Harley decides the best route is to become a bat, stealing Barbara Gordon's costumed identity in Harley Quinn #10.
Disguised as Barbara Gordon's Batgirl, Harley leaps across Gotham taking out anyone trying to claim her bounty. She catches the attention of perplexed locals and even more confused members of the Bat-Family. Years out of the role and acting in her Oracle capacity, Barbara sends the newest Batgirl, Cassandra Cain, to investigate. Cassandra assists Tim Drake during an accidental run-in with Harley and the three begin a fight, with Harley excitedly taking it to the rooftops of Gotham. Despite the numbers not being in her favor, Harley easily disposes of Tim and is able to get the drop on Batgirl by namedropping her secret identity. With the upper hand, Harley blinds Cassandra with her own cape and pushes her off the roof. However, her victory is as short-lived as her Batgirl career, as Cassandra proves her worth by grabbing the copycat's cape and taking Harley down with her.
Once Harley is at a disadvantage, Nightwing, Robin, and Batgirl team up to force her to abandon her new identity, although even then she's able to escape. Years later, Harley's redemption is far further along, and she's now a recognized member of the Bat-Family, fighting alongside Tim, Cassie, and Barbara. The adventure confirmed Harley's incredible competence (since described as on a par with Batman), and established her as a versatile threat separate to the Joker. Harley got the upper hand on the Bat-Family by doing the truly unexpected, defining her role in Gotham and DC Comics going forwards.
Harley's impulsive nature is a double-edged sword. It often gets her into precarious situations, but is sometimes the edge she needs to take on fighters as skilled as Cassandra Cain. She might not be wearing the pointy ears any time soon, but Harley Quinn proved she could be just as effective a Batgirl as Cassandra by nearly taking her out.