Felix could be a main character she interacts with. He is known for his physical strength. In the fanfiction, he might be a training partner for Bella or someone she has to prove herself to. Caius could also be in the picture. He is one of the Volturi leaders and has his own views on things. Bella's presence in the Guard might be a thorn in his side or he might see some potential in her. Marcus, too, although he is often more withdrawn, could still have some sort of interaction with Bella, perhaps through his ability to sense relationships.
Definitely Aro. He is the leader of the Volturi, so he will have a lot to do with Bella.
Caius is another important character in such fanfictions. He is usually portrayed as quite strict and unforgiving, and his reactions to Bella joining the Volturi can range from suspicion to begrudging acceptance. Also, Jane, with her special ability, has interactions with Bella that can be both menacing and oddly fascinating.
Obviously, Bella is one of the main characters. And then there are the Volturi leaders like Aro, Caius, and Marcus. They would play important roles as they are the ones in charge and would have a say in whether Bella can truly join them or not. Also, the Cullens might still be part of the story, especially Edward, as his relationship with Bella could be a factor in her decision to join the Volturi.
In this fanfiction, she might gain new powers. Since the Volturi Guard is known for its strength and special abilities, Bella could be transformed in many ways.
Well, aside from the obvious ones like Alucard, Integra, and Seras, Bella might also interact with Walter. Walter is an old and experienced member of the Hellsing staff. He could be a mentor figure to Bella, teaching her about the history and secrets of the Hellsing Organization. Another character could be the Major from the Millennium Organization. If there's a crossover or a story where the two organizations clash, Bella would surely cross paths with him. His crazy and unpredictable nature would make for some interesting interactions with Bella.
There is also a possibility that a lone vampire, who has a sense of justice or who has been secretly observing Bella and has grown fond of her, decides to take on the Volturi and rescue her. This vampire could have some special abilities or knowledge that gives them an edge against the powerful Volturi.
Caroline Forbes is another character. As a vampire herself in Mystic Falls, she could either befriend Bella or be jealous of her presence. Caroline is known for her strong personality and her relationships within the vampire community.
Stiles is another key character. He's smart and curious, and he would probably be intrigued by Bella's arrival. They could team up to figure out some of the strange happenings in town. Also, Lydia, with her intelligence and unique abilities, could form an interesting relationship with Bella, maybe as a confidante or a rival in some situations.
Bella is a key character. She is usually the one who disrupts the status quo in the Volturi world. Aro is another mainstay. His thirst for power and knowledge makes him a formidable force in the fanfiction. Sometimes, other Volturi guards like Felix and Demetri also become important as they interact with Bella and Aro. And let's not forget Edward Cullen, who might be part of the story as Bella's former love or a rival in some interpretations.
Bella is the central figure in this fanfiction. Then there are the Volturi leaders like Aro. He would play a significant role as he might be the one guiding Bella's upbringing in the Volturi way. Marcus, with his unique ability to sense relationships, could also be important as he might have insights into Bella's development. Caius, known for his strict and often harsh views, would add conflict and tension to Bella's story.
There could also be other vampires or humans who play important roles. For instance, some fanfictions might introduce new vampires who are either allies or enemies of the Volturi and have a connection to Bella's situation. And there might be human friends of Bella who get involved in the story, like Jacob or her school friends, although their roles might be secondary compared to Bella and the Volturi.