In some fanfictions, other villains like Two - Face or the Penguin might be involved. If they are interacting with Harley Quinn or someone the Joker has an interest in, it can lead to the Joker's jealousy. They could be trying to form alliances with Harley or just having friendly conversations that the Joker misinterprets as something more, and that sets off his jealous streak.
The Joker himself is obviously the main character when it comes to his jealousy. Harley Quinn is often involved as she has a complex relationship with the Joker. Batman can also be part of the story in some cases. For example, if Batman is seen as a rival for Harley's attention in a strange way, the Joker might get jealous. Even though Batman is his enemy, the idea of Harley being interested in Batman in any way could trigger the Joker's jealousy.
There could also be original characters created for the fanfiction. For instance, a new female character who is very alluring and starts to gain the attention of the Joker's crew. If the Joker feels that his influence over his crew is being threatened by this new character's charm, he will get jealous. And this new character, along with the Joker, Harley Quinn and potentially other villains, would be the main characters in that particular fanfiction.
Chapter 317, "You Can't Afford to Mess with the Person Behind Her!", is the clearest early answer because it turns Mess's situation into a concrete plot event. It opens with He Pingzhang gazed at the man curled up before him, unable to... and closes on When had he offended such a character?!.
Chapter 178, "The Group's Favorite Belongs to No One but Her!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Wen Jiaojiao is just too unreasonable, why does she have to single...
Chapter 38, "Pretending to be a Top Student? You Must Have Forgotten Your Own College Entrance Exam Score!", is the clearest early answer because it turns Princess Disease's situation into a concrete plot event. Early on, Jiang Li had also observed the girl who had just entered..
Chapter 170, "This Family Will Fall Apart Without Jiang Li", is the clearest early answer because it turns Chixia Village's situation into a concrete plot event. The chapter turns that into something concrete by moving from Watching the two who had long remained silent, the PD shifted his...
Chapter 17, "Madam, the eldest young master has disappeared again", is the clearest early answer because it turns Aunt Zhang's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Jiang Li glanced at Jiang Shixu, who was tense and anxious behind...
Chapter 276, "He Will Definitely Make This Woman Pay a Price!", is the clearest early answer because it turns Zhou Xu's situation into a concrete plot event. It opens with After listening to Jiang Li's words, Shen Lan's brow furrowed again.
Chapter 55, "I Can Prove Everything Miss Jiang Said Is True!", is the clearest early answer because it turns Miss Jiang Said's situation into a concrete plot event. Early on, Su Yinwan hadn't expected that, in just one day, Jiang Li's.... By the end, He Yun's fans were immediately displeased..
Chapter 13, "Killing the Chicken to Scare the Monkey", is the clearest early answer because the violence around Mrs becomes final instead of hypothetical. The chapter turns that into something concrete by moving from Sensing that something was off with Jiang Yan, Jiang Yan dismissed...
Chapter 375, "Jin Siyue and Dogs Not Allowed Entry", is the clearest early answer because it turns Jin's situation into a concrete plot event. The chapter gives readers a real answer by opening on He Pingzhang spoke eloquently, but the moment he opened his eyes,...
Chapter 8, "Is He Shameless?", is the clearest early answer because it turns Jiang Lili's situation into a concrete plot event.