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.
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.
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.
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.
Perhaps there are characters from outside the pack that also play a role. Enemies of the pack could be main characters as they pose a threat to Bella and the pack. This would give Bella the opportunity to show her protective side as the pack mother. Or there could be some neutral characters who get involved in the pack's affairs because of Bella's new role and end up being important to the story.
A doctor named Dr. Smith could be a main character. He could be a strict but fair physician who at first doubts Bella's abilities but later comes to respect her.
The Cullen family members are important characters too. Edward, for example, might have an interesting dynamic with Bella in this fanfiction. He could be either protective or jealous of Carlisle's special relationship with her.
Alice could be a main character. Since she can see the future, she might sense something special about the new coven and be the first to approach them with Bella. Her insights could be crucial in understanding the new coven's intentions.