I'm kind of a nerd.

Joker seduced Harley while he was in Arkham and she was his psychologist. He did so by manipulating sessions to make him seem pitiable.

Harley broke Joker out. Joker was originally going to kill her then, but fans had latched onto Harley Quinn’s new look and she was a fan favorite (mind you as I recall, she was originally introduced in BTAS, and then transferred to the comics later). So she ended up surviving his first murder attempt.

He decided that although annoying she could still be useful (since she’s actually brilliant, and at this point somewhat codependent). This leads to a string of horrific abuses and murder attempts. Including (in the TV show alone) throwing her through a window that is at *least* three stories up, choking her, beating her with a hammer, threatening her with one of his gag guns (which, depending on the gun, may or may not kill her in various ways), and attempting to get hyenas to eat her.

In the comics, it includes starving her, chaining her to a wall in a sewer on top of corpses of “failed Harleys,” poisoning her, leaving her in burning buildings, pushing her into the line of police fire, gaslighting her basically every time he fails to kill her, and the list goes on. When she becomes pregnant with her and Joker’s kid, she leaves for nine months, to her sister’s place, and gives birth there. She doesn’t tell Joker about the kid (and goes out of her way to prevent Joker from finding out). She tells Canary that it’s because Mr. J would be too busy for a kid, but if you pay attention to Harley’s behavior throughout the comic, the clear subtext is “My kid would end up dead or worse if Joker knew about her.”

When I heard there might be robot romances in Fallout 4: