When Harley Quinn was introduced to the world in 1992, she was the henchman of the Joker. By the next time she showed up, it became obvious that she and the Joker were in a relationship, though what exactly that entailed was a bit mysterious — outside of tormenting Batman together, that is.
Harley and Joker have one of the most notoriously toxic relationships in all of comics history, and one consistent theme in Harley's life is that she is always better off when she's away from the guy. Yet, for anyone that has gone back to a bad relationship searching for something that just isn't there, Harley's story is painfully relatable. She is the epitome of the "don't text your ex" rule.
In Gotham Sirens #7, published in 2010 and written by Harley co-creator Paul Dini, we learn that Harley is from an abusive household when she goes home to visit her family at Christmas. Her father is a charming villain, much like the Joker, who manipulates women into taking care of him. Her mother is full of hostility and resentment towards everyone including Harley, and her brother has children he does not care for. We discover that Harley became affiliated with psychiatry to understand the inner workings of her family's dysfunction, only to find an unhealthy fixation of her own.
In 1995's Mad Love, the story of how she became involved with the Joker finally came to light. We see Harley as an inexperienced psychiatrist who is thrown into the same room with one of the most skilled manipulators in fiction. She quickly falls under his spell and sacrifices everything to help break him out of Arkham. The Joker does not reciprocate her love and is in fact incapable of romantic emotion as he sees people only as tools to further his own plans. Harley is in denial of this fact and prefers to view him as an antihero until she eventually has to come face-to-face with his complete disregard for her.
Though Harley was indeed a cold-blooded killer throughout this time period, there is no doubt that she also underwent a lot of intense abuse at the Joker's hands. She believed herself in need of breaking so that she could be reformed as what the Joker wanted her to be, but even when that transformation was complete and she'd given up everything, it wasn't enough. This was only a fraction of the gaslighting and abuse that she went through in this relationship. On the other hand, the Joker regularly planned for her death, undermined her, preferred her to act unintelligent "for humor," and lied to her face. When the Joker was gone, Harley was listless and upset while her absence seemed to have almost no effect on the Joker at all.
In the Arkham Asylum video games, Harley was just as painfully obsessed with the Joker as ever, while the later Injustice timeline saw her making peace with their separation as she tried to do better things with her life. Still, Harley was regularly in the Joker's shadow during her initial character arc, and even with all her growth, there have been times when she went back to him. In her first ongoing comic series, she teams up with Poison Ivy and moved to Metropolis, standing in at the Daily Planet as an advice columnist. Though she ends up dying later in the series, it's not permanent, and she does grow by leaps and bounds even if it is ultimately undone by the company-wide retcon of the New 52.
A survivor of Jason Woodrue, Poison Ivy had firsthand experience in leaving an abuser, and she supported Harley through her process, beginning with Batman: The Animated Series all the way to the present day. While their relationship changes context regularly, from partners in crime to best friends to lovers to wives and back again, Ivy's willingness to meet Harley where she's at while staying honest and absolute about her disdain for the Joker makes her an ideal counterpart to Harley in an emotional sense. Besides, they have perfectly matching color schemes and share an overriding interest in STEM. It's easy to see that Harley and Ivy have always had more in common than Harley and Joker ever will.
Currently, even in the animated series and film franchises, Harley has broken away from the Joker and has made her own way. Her relationship with the Joker was unsustainable, to say the least. It seems that even her creators knew the pairing would quickly become old hat, as they separated them within the animated series as well. In the Injustice and Bombshells universes, we see Harley at her very best with Ivy, while in all possible futures with the Joker she is miserable and ultimately is physically destroyed by her love for him.
In life, it would be incredibly difficult to get past Harley's many crimes, but characters from Big Two superhero comics are constantly changing, crossing moral event horizons, and coming back from them again. As the world embraces a more heroic Harley Quinn, her actions will be placed into a different light. Though some attempts have been made to sympathize with the Joker, he was established as an unrepentant killer as early as 1940, and it's more difficult to find sympathy for him as a result.
It's a struggle to walk away from intense and passionate but ultimately toxic love affairs. Loads of research has been done around why people stay in these love affairs long-term, and though the commentary does not always succeed, Harley was an attempt to address that. Importantly, Harley always gets away from her toxic beau in the end. Regardless of anything, the Joker has an incredibly negative effect on Harley, and while he may have been his best self with her, she was always her worst self with him. The demise of this relationship gave Harley what a lot of people get when their abusers are no longer active in their life: a happier story.