Harleen Quinzel was Joker's psychologist. Her therapy sessions with him slowly sent her insane and caused her to fall in love with him.Shortly after, she donned the original Harley Quinn uniform and joins the Joker as his sweetheart and partner in crime. She was later approached by Task Force X.
- The cartoon that Harley is seen eating during the opening sequence is the opening of The New Looney Tunes Show, another creation of Warner Brothers.
- The sex scene Deadshot and Harley Quinn have was taken from the DC "New 52" comics, where they were both members of the Suicide Squad and did the same thing.
- When Harley Quinn is about to rob a toy store she puts her head against the glass very much in the same fashion as Catwoman did in the movie.
- When Harley goes through items in the Joker crate at Arkham, among the items she removes is the mask that the Joker wore in the opening scene of The Dark Knight(2008), and a Joker fish from Batman: The Animated Series (1992).