When Dr. Harleen Quinzel abandoned her life as a psychiatrist for Arkham Asylum to live as a supervillain alongside the Joker, she took to it like a duck to water. While she excelled in reading and diagnosing people psychologically, Harley Quinn turned out to be even better as a costumed villain in Gotham City. While Harley Quinn doesn't have powers in the traditional sense, she's shown an amazing ability to think on her feet and improvise solutions whenever backed into a corner. If there's one thing that can be said of Harley, it's that she knows how to turn any new situation to her advantage.
And that's a good thing because Quinn was thrown for a real loop after she found herself a spirit without a body. In Harley Quinn #24 by Karl Kesel and Craig Rousseau, Martian Manhunter discovers that Harley is on a body-jacking spree after he realizes that his partner is currently possessed by her. After Harley Quinn was banished from Hell, she couldn't return to her body as it had been destroyed in Metropolis. So in lieu of her own body, Harley takes over the bodies of others with weakened or absent psyches. She even reveals that before she took over Martian Manhunter's partner, she tested out her newfound power possessing a comatose woman. Harley reveals that she wanted to "fix" the woman's life by divorcing her husband and pairing her up with her chauffeur, something that angered the woman so much, she stole back control from Harley and killed herself.
Thankfully Martian Manhunter requests his comrade Zatanna use her magic to stop Harley’s possession spree. But for a brief period, Harley got the upgrade of a lifetime (despite not technically being alive). The more Harley possessed others, the better she got at it. And if left unchecked, there's little doubt that what happened with Quinn's comatose victim just would have been the start of a terrifying reign.
One of Harley’s biggest Achilles Heels is her devotion to love. It’s what kept her trapped in an abusive relationship with the Joker for so many years and it’s what got her in trouble with her first possession victim. Harley Quinn’s idea of love is unconventional and one of her biggest flaws is that she tries to force that idea onto others. On her own, that's no big deal. With the powers to possess others and take control of their lives? That's an instant nightmare scenario. Just as she ruined the life of a comatose woman, Harley could flit about from body to body, causing chaos by playing a demented game of Cupid and putting people together with matches that fit her irregular form of love. Thankfully the League resolved this issue before Harley Quinn’s powerful upgrade became permanent.