Well, in this type of fanfiction, Rosalie would definitely be involved. Given her past feelings towards Bella, her reaction to Bella becoming a vampire could range from begrudging acceptance to outright hostility. Emmett, too, as Rosalie's partner. He might be more easy - going about Bella's transformation. And don't forget about Esme. As the mother figure in the Cullen family, she would likely play a role in helping Bella adapt, perhaps teaching her how to blend in with humans despite her new vampire instincts.
The Cullens are very likely to be involved. Edward, of course, would have a huge role as Bella's former love interest. Carlisle, with his knowledge and experience as a vampire, could help Bella adjust to her new life. Alice might use her precognitive abilities to foresee any problems Bella could face. Also, Jacob could be in the story. His reaction to Bella becoming a vampire would be interesting, considering his past relationship with her and his werewolf nature.
Well, it could be the people she targets. They would be important as they drive the action when Bella is on her 'killer missions'.
Edward could be one of the characters. He might be in a love - hate relationship with Bella due to her addiction. He could be torn between his love for her and his inability to stand her self - destructive behavior.
Foster parents are likely to be important characters. They could be kind and nurturing, or perhaps they have their own issues that create more drama for Bella.
Edward is almost always there. Since he's such a central figure in Bella's life as a human and vampire in the original story, he would likely play a big role in the fanfiction too. He might be the one guiding Bella through her new vampire life.
Edward is still very popular. Since he and Bella are now both vampires, their relationship takes on new aspects that fans like to explore. Another popular character is Alice. Her ability to see the future could be really interesting in a story where Bella is a vampire. She could foresee events that involve Bella and help her avoid danger or make important decisions.
Jacob could also be a common character. Even though Bella has become a vampire, her past connection with Jacob and the werewolf pack might still play a role. There could be some tension or new interactions between the vampires and the werewolves. And of course, Carlisle, as the head of the Cullen family, is likely to be in many of these stories to guide Bella in her new vampire life.
If it's set in a school or work environment, Bella's friends or colleagues could be main characters. They might notice the changes in Bella as she discovers this new family connection. Also, if there are antagonists in the story, it could be someone who doesn't want Bella and Billy to form a relationship. This could be someone who has a grudge against Billy and sees Bella as a way to get to him, or it could be someone who is jealous of Bella getting a new family.
There could be a close friend of Bella's, like Alice. Alice might be someone who starts to suspect that something is wrong with Bella but Bella keeps pushing her away. Another character could be the abuser, who could be someone in a position of power over Bella, such as her boss at work or a teacher if it's set in a school - like scenario.
Another character that could be involved is Charlie, Bella's father. His reaction to his daughter being pregnant in the strange world of Eclipse could be a significant part of the story. He might be confused, worried, or try to protect Bella in his own way. Also, some of the werewolf pack members aside from Jacob could also make appearances and contribute to the story based on their relationships with the vampires and Bella.
There could be a female character, Sofia, who is another member of the mafia. She sees Bella as a threat at first, thinking she'll steal her position or the attention of the men in the mafia. But as time goes on, they might form an unlikely alliance. There may also be a young and ambitious mafia underling named Antonio, who is always looking for ways to climb the ranks and his actions often impact Bella's situation in the story.