Dr. Harleen Frances Quinzel (also known as Harley Quinn) is a villainess from the Batman series and one of the many villains of DC Comics.
She was the Joker's self-proclaimed girlfriend and henchgirl. She was created by Paul Dini and first appeared in his Batman cartoon.
According to the comic book canon, she was interning at Arkham Asylum as a psychologist when she met the Joker. She got overly fixated on him and started to believing that she loved the Joker.
She does whatever she has to do to achieve the Joker's approval. She also allies herself frequently with Poison Ivy.
Depending on the writer and story, Joker either doesn't love her and just manipulates her into doing what he says or he can love her but is just very abusive.