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.
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.
Well, often it's Clary, Jace, and Simon. Simon is Clary's best friend, and in some fanfictions, his relationship with Clary can make Jace a bit jealous, but at the same time, if Simon seems too friendly with another girl, Clary might also get jealous in a different way. It's an interesting triangle - like situation that can add a lot of drama to the fanfiction. And of course, Magnus Bane might also make an appearance and his flamboyant personality could somehow be involved in situations that lead to Clary's jealousy.
Obviously, Naruto is the central character. His relationship with the Nine - Tailed Fox is the core of this kind of fanfiction. The Fox itself, which is a powerful and mysterious entity within Naruto's body, is also a key character.
Well, the main characters are obviously Bella and Edward. Bella is the one who tries to make Edward jealous, usually by getting close to other male characters. Edward is the one who gets jealous because of his deep love for Bella.
Obviously, Mal is the central character. But often, her friends or romantic partners can be important characters too. For example, if she has a significant other, they will play a big role in how she copes with the pregnancy.
Dumbledore can also be a significant character. He might be the one who decides Harry's punishment or he could be the one to offer some wisdom to Harry during his time of being grounded. And of course, the teachers like Snape or McGonagall could be involved as they are often the ones who enforce the rules and punishments at Hogwarts.
Well, of course, Harry is the central character as he is the one who gets turned into a girl. His friends like Ron and Hermione are also usually very important. Ron might have a really different dynamic with Harry now that he's a girl, and Hermione could be there to support Harry through the whole transformation process.
In most 'Joker Naruto' fanfiction, Naruto is always there as the hero trying to protect his village and his way of life from the Joker's antics. The Joker is the main source of trouble. But characters like Sakura can also be important. She might use her medical ninjutsu to heal the victims of the Joker's chaos. And then there are the Hidden Leaf Village elders. They would be worried about the impact of the Joker on their village and might try to make decisions on how to handle the situation, making them significant characters in these stories as well.
Obviously, Naruto himself is the main character as he is the one sent back in time. Sasuke is also often involved as Naruto's relationship with him is a central part of the Naruto story. Kakashi could be involved too, as Naruto might seek his help or try to influence his training methods. Also, the various Hokages might be important characters as Naruto could interact with them in the past to change the course of events.
There can be original characters created specifically for the fanfiction. For instance, a detective who is obsessed with bringing down the Joker but slowly starts to lose his own sanity in the process. This character can provide a new perspective on the Joker's reign of terror.
In the fanfiction, perhaps there's a new recruit in the Joker's gang who is very talented and starts getting a lot of attention. The Joker, being used to being the center of attention, gets jealous. He might start giving the new recruit impossible tasks to do in order to undermine them and regain the spotlight.