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.
Bella's family members like Charlie could be important characters. Charlie might be heartbroken to discover his daughter's addiction and struggle to find ways to help her, while also dealing with his own feelings of guilt and failure as a parent.
Jacob could also be present. He might try to be the more practical friend, trying to get Bella to see the error of her ways and get help. He could be a source of support or a source of conflict depending on how Bella reacts to his attempts.
Well, it could be the people she targets. They would be important as they drive the action when Bella is on her 'killer missions'.
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.
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.
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 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.
There could be Edward. Even if Bella is now immortal, Edward might still play a significant role. He could be either her eternal love or her adversary depending on how the fanfiction is written. Another character could be Alice. Since Alice has her own special abilities, she might be there to help Bella navigate the new world of immortality. For example, she could foresee some of the challenges that Bella will face. Also, Jacob might be a main character. His relationship with Bella, whether it's a friendship or something more complicated, could be further explored in the context of Bella's immortality.
There could be Edward. Maybe he is involved in Bella's life with her psychic powers. He might be curious about her abilities or even try to protect her from the consequences of using those powers.
One common theme could be the struggle of Bella with her addiction. It might show her inner turmoil, her attempts to hide it from friends and family, and the downward spiral it can cause in her life.
There could be her parents. They might play a role in encouraging her to get braces or helping her take care of them. For example, they could remind her to brush her teeth properly with braces.
There could be her drill sergeant. This person would be tough and no - nonsense, pushing Bella to her limits during training. Another character might be her best friend in the air force, someone who is always there to support her, through the good times and the bad. There could also be a rival, someone who is constantly competing with Bella for promotions or recognition.