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.
While Bella and Edward are the main focus, sometimes the Volturi are mentioned in a more distant sense. Their presence in the overall Twilight world looms over the couple, but on the wedding night in fanfictions, it's more about Bella and Edward's personal and private moments. It's about their love, their dreams, and their new life together as husband and wife.
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.
Obviously Jacob is a great choice. His friendship with Bella always had the potential to make Edward jealous. Then there could be a new vampire, like a very sophisticated and handsome one who takes an interest in Bella.
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, 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, Edward and Bella are the central characters. Their relationship is at the core of these fanfictions. But other characters like Jacob also play important roles sometimes. Jacob might be there as an on - looker, with mixed feelings of jealousy and acceptance. He could add some drama to the story, especially if he still has some unresolved feelings for Bella.
The Cullen family usually plays a significant role. Carlisle, with his medical knowledge (even if it's vampire - related in the Twilight context), might be involved in ensuring a safe birth. Alice could be there, perhaps using her gift to foresee any potential problems during the birth. Even the other members of the Cullen family would be around to offer their support in different ways.
Obviously, Bella Parker is the central character. Often, her friends from the performing arts world, like her dance classmates or instructors, are also important characters. They might be there to support her, compete with her, or collaborate on various projects.
One aspect could be that in such fanfiction, jealousy often adds a new layer of drama to the relationship between Edward and Bella. For example, if a new character is introduced, and either Edward or Bella shows jealousy, it can lead to interesting conflicts. It might also explore their insecurities within the relationship more deeply.
Maybe Bella goes to a party and dances with a lot of other guys. She acts really carefree and happy with them, ignoring Edward. This would make Edward seethe with jealousy. He might start to question their relationship and become a bit more withdrawn, which in turn could make Bella realize she's gone too far.