Harley Quinn is the Joker's on-and-off girlfriend, and possesses acrobatic skills along with enhanced strength and speed thanks to her friend Poison Ivy.
Stephanie met Harley Quinn in Joker: Last Laugh (3), when she and Cassandra were sent by Oracle to save The Joker's girlfriend from a jokerized Phobia and other cohorts. Harley referred to our dynamic duo as "the campfire girls" and all three of them ended up getting captured, but Oracle dialed "1-800 Power Girl" to rescue them. Power Girl took Harley away.
Learn more about Harley here