WARNING: The following contains spoilers for Batman: White Knight Presents Harley Quinn #2 from Katana Collins, Sean Murphy, Matteo Scalera, Dave Stewart, and AndWorld Design, on sale now.
In the Batman: White Knight Universe, it's been a secret as to what truly and totally destroyed Jack Napier's mind and allowed the Joker to take over. Batman: White Knight Presents Harley Quinn actually shows them as a normal couple of years ago, which brings up the question of what the catalyst for him to turn was, become abusive towards her, and embark on his acts of terrorism.
Well, Issue #2 finally reveals the breaking point, and, shockingly, Harley has herself to blame as some carelessness on her part, coupled with Jack's insecurities, created this Clown Prince of Crime.
Jack was a typical mob goon, hanging out at strip joints and running from the Bat. It's here that he met Harley, but they decided to start a romance as their chemistry was too strong to ignore. It turns out, as Harley gets intel from Neo-Joker at Joker's derelict amusement park, they were a pretty normal couple as they were going on carnival dates, spending time together, and plotting a future where they moved in together.
However, Jack's split personality started to get worse and worse, and as Harley continued to document the dementia as a psych student, she thought it'd lead to a cure. She was his True North, helping bring him back to the light but the fits were becoming more prevalent and very harsh. Unfortunately, after graduating with accolades, Harley published a paper that got Jack irate as he felt a sense of hopelessness within.
He felt like her guinea pig, a lab rat she was studying and in an emotional argument, he couldn't see she was trying to help him. He felt used and like Harley was trying to build her career off his condition, which caused him to run away for a bit. Harley was crushed as she shouldered the blame, only for the Bat to visit her and reveal Jack was going off the rockers. This insinuates that in this downtime, he'd transform into the Joker, with the Dark Knight telling her if she really wanted to continue treating him, enroll in Arkham Asylum so she could attend to him as a patient, not a lover.
Unfortunately, Harley couldn't save him, or separate professional life from personal life. Only Batman knew of the conflict of interest. And it would result in this chaotic partnership evolving because Harley felt guilty for Joker's devolution, thus why she joined his crusade. She became his right-hand woman but also his caretaker, as he became psychotic.
She didn't care about sacrificing her career, family or friends as she truly loved and hoped Jack would return someday. But in the meantime, she enabled Joker in a situation that is admittedly a bit sympathetic. We understood Jack snapping as it was all about him feeling like the only person he ever trusted abandoned him, while Harley merely wanted to protect the man she loved. It all came down to bad communication and as Harley now works with FBI agent Quimby and Neo-Joker to catch a copycat killer, it's clear this origin story still haunts her.