'thumb|333px|link=File:Harley_Quinn.png'Harley Quinn a.k.a. Dr. Harleen Quinzel (voiced by Arleen Sorkin) is a villainess who first appeared in Batman: The Animated Series and has become a big villain in the Batman universe. She was originally a psychiatrist at the Arkham Asylum prison. She fell for her patient, The Joker, and usually helped him escape from prison. But once the Joker is returned injured after a fight with Batman, Harleen goes insane (or, as Bugs would put it, "She done went screwy!") when she sees her injured patient, and dresses in a jester outfit to become Joker's accomplice and girlfriend.
Harley is one of the Bugs and Daffy's Adventures gang's biggest enemies, along with The Joker. Apparently, on the Warner Bros. set, the Looney Tunes (mainly Daffy, along with Yakko, Wakko, and Dot) drove the actors playing the two on the Batman show to go insane and become their roles. Now, along with Marvin the Martian, Yosemite Sam, and Elmer Fudd, they plot revenge against them.
- The Bugs and Daffy's Adventures gang first tangles with Harley in Bugs and Daffy's Adventures of The Emperor's New Groove.
- Harley Quinn will become Jaden and Jeffrey's enemy in Jeffrey, Jaden & Friends' Storm Adventures of Batman: The Animated Series.
- Harley Quinn is Cleo's older sister.
- Harley will return with Joker in Xion's Ohana Adventures of 102 Dalmatians.