Warning: spoilers ahead for Batman #107, Harley Quinn #1, and Man-Bat #3!
Fans of the dynamic femme fatale Harley escaped DC's Trinity in Heroes in Crisis - but she also possesses a useful set of skills that the Dark Knight lacks. Her time spent as Joker's sidekick/love interest hid that behind a seemingly empty-headed persona, but all the while, Harley was learning the ins and outs of Gotham's underground crime scene without knowing it would lead her to being one of the top crime-fighting heroes DC has ever seen.
Though she has spent the majority of her time leading teams and single-handedly catching criminals as of late, Harley still tends to be inaccurately given the "sidekick" label. Many see the former Cupid of Crime as a less qualified right hand woman to the iconic Batman. Though she does refer to herself as "Batman's official sidekick" in Batman #107, this issue also proves that Harley brings a whole new perspective to the table, and it's one that gives her an edge over even the Dark Knight himself.
It stands to reason that Harley knows Gotham's crime scene better than anyone. Her time spent as Joker's girlfriend exposed her to plenty of villains who have since become her enemies. Batman and other heroes may feel like they know the killers and thieves they have been fighting against for years, but Harley truly knows them. By working with them in the past as a villain, she has learned their strengths, weaknesses, tactics, patterns, hideouts, and even minions, all with the insight of a forensic psychiatrist. Stephanie Philips and Riley Rossmo's Harley Quinn #1 shows Harley effortlessly track down Killer Croc, while Future State: Harley Quinn saw Harley forced to cook up plans to take down Gotham's rogues, each of which worked perfectly.
Still, her connection to Gotham's underground is not all that sets her apart. Batman and Harley have completely different approaches to catching criminals, and define a successful mission in different ways. Batman's goal is to prevent any harm coming to innocents, stopping criminals with extreme measures and scaring or injuring them badly enough to keep them off the streets. Bruce Wayne is bound up in his own childhood trauma, seeing crime-fighting as a war, but Harley has a far more rehabilitative approach, and the perspective not just to recognize the varied motivations of Gotham's criminals, but to adjust her tactics accordingly.
This is seen in both Batman #107 and Man-Bat #3, where she risks her life as a member of the Suicide Squad to offer Batman's bestial villain some therapy and set him on the right track. Where deep down Batman wants to punish criminals, Harley doesn't have the same urge - if anything, her instincts run in the opposite direction, as she attacks a cop who's holding her at gunpoint while she tries to help displaced Arkham inmate "Stabbo." The cop has a similar approach to a lot of heroes in the DC universe, focusing on bringing down the villain in order to protect innocents. Harley would rather protect the criminal as well, but has her own bias in how quickly she loses her cool in the face of aggressive authority.
Harley's partnership with Batman should be an interesting one to say the least. Though they may have different connections, as well as different approaches, they are both incredibly dedicated to the crime-fighting life. It's possible the two will even learn important lessons from each other. Batman has been known to shortcut around the diplomatic approach in favor of righteous vengeance, but Harley Quinn's psychiatric insight, firsthand experience of Gotham's underworld, and less fanatical view of crime mean that in cases where escalation only leads to more trouble, she has a huge advantage over the Dark Knight.
