The Joker's influence on Harley Quinn has been clarified all along, but his influence may have jeopardized the fate of the multiverse.
Arguably, the Joker and Harley Quinn had (if not, still have) one of the most painfully twisted "romances" in DC history. For the longest time, it was misunderstood by Halle herself, who often saw her beloved "Mr. J" as a lover and confidante who knew her better than anyone else. The Joker only sees Harley Quinn as a punching bag and a maid who does his dirty work. To make matters worse, after realizing the love she's always harbored for him, the Joker will use this to his advantage, convincing Harley to do anything for him while they're together.
Harley didn't see the light of day until the last decade. When Harley Quinn transforms herself from hapless villain to hero, she finally sees the Joker's true mistreatment of her. In the process, her unquestionable love for J-Man quickly turns into pure hatred. But the main DC Universe's Harley Quinn isn't the only one forced to put up with the threat of the Joker.
Harley Quinn Reveals That The Joker's Abuse is a Multiversal Constant.
Stephanie Phillips, Matteo Lolli and David Baldeon's Harley Quinn #25 is slowly being unveiled, The Harley Queens of the entire multiverse are tormented by the Joker's twisted notion of love. One of the Harleys in particular (let's call her Evil Harley) is tasked with killing every version of Harley Quinn she can find in the multiverse. As Harley and Mrs. Harley hypothesized why this evil Harley wanted to kill every Harley in the multiverse instead of saying, "Mistah J", they spent years verbally/emotionally abusing them, saying the Joker "made me feel like I'm the villain even when I'm not around. Makes me feel like your present can't outlast your past."
Seems like this evil Harley might be so hurt by the Joker's influence that she blames herself, Desperate to destroy every possible version of yourself as a punishment you inflict on yourself. It's clear that spending years with the Joker has had a detrimental effect on Harley, but this demonstrates that the Joker's influence is actually a multivariate constant in the DC Universe.
The multiverse constant means that any thing, person or moment, regardless of differences, is destined to affect each universe. For example, in the past, Superman was called Nightwing A multiverse constant because he's calm and confident in every universe he's encountered. With every version of Harley, the Joker is always destined to treat her horribly and disrespect her. It's a mixed blessing to think that the Joker's torture of Harley Quinn not only affects her across multiple universes, but big enough to affect the fate of the entire multiverse.
Next: Harley Quinn is Redeemed by a Truly Shocking Hero