This can be traced back to the original Batman: The Animated Series and has continued through to current stories. One of the main reasons why Harley Quinn loves the Joker is due to his destructive personality. In one instance, the Joker set everyone in Arkham Asylum free and then blew it up.
Historically, Harley Quinn is a former psychiatrist who falls in love with the criminally insane Clown Prince of Gotham. Joker manipulates Quinn and she falls for his devilish charm, not knowing that it would be her eventual downfall and she would become a partner in clown-themed crime with Joker.
During their sessions, she became obsessed with the criminal and helped him escape the facility. Madly in love, she created a new identity for herself as Harley Quinn, sticking by the Joker's side throughout all of his sick adventures.
This was all explained in The Batman Adventures: Mad Love (1994) graphic novel, which was a tie-in to Batman: The Animated Series. The story expanded on Harley Quinn's animated role and explained how she fell in love with the Joker; the graphic novel was widely praised and received several awards.
Joker Jr. was born after The Joker, who had manipulated Harley Quinn into re-entering a life of crime, impregnated her without her knowing. Harley out of shame refused to accept the child and gave him up.
Believe it or not, this tale saw the Clown Prince of Crime himself become pregnant and give birth to a child. The mother of the child? That was none other than DC's famous magical hero Zatanna, who cast a spell on the Joker after he tried to persuade her to start a family with him.
However, there has been one green spot of light in her love life in the form of Pamela Isley, Poison Ivy. And in Batman: Urban Legends #1, Harley confirms that no matter her turbulent past with the clown, Ivy really is the one true love of her life and the soulmate she wants back.
Harley Quinn has had several love interests, the most notable being the Joker and Poison Ivy. Other love interests include Mason Macabre, a character created by Conner and Palmiotti. Plastic Man was initially intended to be a love interest for Harley Quinn by Kesel, but was not approved by DC.
Harley Quinn is the Joker's most famous accomplice and lover. They met when she was a psychiatrist studying mentally ill criminals. When she decided to analyse the Joker, the young psychiatrist sealed her fate.
The first is the Joker who, with his influence, led Harley into the tumultuous world of villainy, changing her life forever. The other is Poison Ivy, a bond that started as a close friendship before blossoming into a relationship Harley Quinn found genuine love and appreciation in.
As Harley recalls, she had ordered some pudding that night, a choice that surprised Jack based on all of the other food varieties offered there. Harley playfully retorted by explaining that pudding was classic, just like him, to which she proceeded by saying his ubiquitous nickname.
The blue and red hair dye was in Suicide Squad first. In the comics she was blonde until the New 52, where she started dying her hair red and black. Then for her New 52 series she was blonde until she decided to do the pink and blue tips ala Margot Robie's Harley.
David Ayer's Suicide Squad, however, would mark the first live-action movie to feature both characters and the origins of Quinn's relationship with Joker.
COMIC LEGEND:
Harley Quinn calls the Joker "puddin'" because he gave her a pudding cup when they first met.
Punchline: Joker's Upgrade
Her true name is Alexis Kaye but calls herself Punchline. She is cold and sadistic, nothing like Harley Quinn. Alexis Kaye is introduced as a student who is infatuated with Joker.
Jeannie was the pregnant wife of the Joker.
In addition to Harley and Punchline, other women have loved the Joker in other universes where he terrorizes Gotham City, including characters such as Neo-Harley. But all of these women met Joker when he was Joker. None of them got to know the man before he turned into the Clown Prince of Crime.
Personality Disorder, specifically, Histrionic Personality Disorder plays a key part in Harley Quinn's life. People with Histrionic Personality Disorder are “pervasive and excessive emotionally and display attention-seeking behavior” (Bornstein 1998).
Harley Quinn features Ivy as Harley's best friend and then lover.
Batman: Beyond the White Knight #2 ended with the surprising revelation that Bruce and Harley had tied the knot following the events of Batman: Curse of the White Knight.
In Gotham Adventures #29 Ivy gets physical. She strangles Harley and wrestles with her, and calls her names. She is only barely tolerant of her and her treatment of Harley is as rough as anything we've seen the Joker do - including the name-calling.
Penny had a mental disorder and her case history records at the mental health clinic mention Arthur having a head injury after she abused him, tied him to a radiator and beat him alongside her abusive boyfriend.
The ending of Batman: Three Jokers establishes that the Joker's wife did not actually die—rather, she fled to Alaska with the help of Gotham police and Batman because she feared her husband would be an abusive father; the police then told the Joker a story about her dying to protect her.
In Countdown Presents The Search for Ray Palmer: Crime Society it is revealed that Earth-3's heroic version of the Joker, the Jokester, is the father of Duela Dent and that her mother was Evelyn Dent, Three-Face (the Earth-3 version of Two-Face).