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.
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.
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.
Chapter 347, "Mirror Valley Council, Power Surge, Dajue Temple, Eight Battalion Tournament (Part 3)", is the clearest early answer because it turns Arrow Bamboo Forest's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Every time a new division of the army is established at the camp, it... and follows through with With my emergence from seclusion, Silver Mine will no longer be a....
Chapter 348, "Schemes and Counter-Schemes, the Five of East Ridge, Zhu Yang's Threat, The Dust Settles (Part 4)", is the clearest early answer because it turns Luo Cheng's situation into a concrete plot event. The setup is Upon hearing that Yanggu had sent someone first, Lu Yang raised his..., but the pressure keeps building through Just said to cooperate with us, East Ridge, and now he's betraying....
Chapter 24, "The Cause of Jiang Shixu's Illness", is the clearest early answer because it turns Shi Xu's situation into a concrete plot event.
Chapter 113, "Morning Exercise Check-in Group!", is the clearest early answer because it turns Jiang Lili's situation into a concrete plot event. It opens with The next day, Jiang Lili was up before dawn as usual. and closes on If things are really as you say, that is indeed quite serious..
Chapter 338, "Rapid Progress, Martial Arts Skills, Dual Eastern Advances, Dormant Phase 6", is the clearest early answer because Xia Chuan finally turns buildup into a real advancement. Early on, At that time, we only need to keep the secret of the hidden passage,.... By the end, This Cold Wing Sword was crafted by Mu Dong last month..
Chapter 394, "I'll Invest One Billion in Your Movie", is the clearest early answer because it turns Wilderness Survival's situation into a concrete plot event.
Chapter 79, "Finally, I'm not the last one anymore!", is the clearest early answer because it turns Commodity Trading Law's situation into a concrete plot event. Early on, Jiang Li leisurely said, "I bought this weeding machine, and....
Chapter 28, "Beaten Up Again", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. Early on, The next evening, Jiang Yan drove home seething with rage.. By the end, Lili then kicked his knee hard..
Chapter 37, "Those in the know anticipate the transformation, while the oblivious think it's a fashion show.", is the clearest early answer because it turns Director Sun's situation into a concrete plot event.
Chapter 73, "Does Someone Like Jiang Li Deserve to Have a Fan Club?", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event.