1) Harley Quinn was originally Harleen Quinzel, a psychologist who worked at Arkham Asylum and fell in love with her patient, the Joker.
2) Harleen gradually took on the Joker's personality and became his accomplice Harley Quinn. She displays an extreme attachment to abusive men like the Joker and Poison Ivy due to likely childhood attachment issues.
3) Harley exhibits traits of antisocial personality disorder, conduct disorder as a child, and anxiety disorders which were possibly caused by childhood negligence from her parents. Her life trajectory could have been altered with parental intervention as a youth.