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.
One possibility is that Bella is forced to join the Volturi for some reason. Perhaps the Cullens are in trouble and the Volturi offer Bella a deal - join them and they'll spare the Cullens. So she enters their world, where she has to deal with the different personalities among the Volturi members, like Aro's calculating nature and Caius' harshness.
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.
One great 'bella joins volturi fanfiction' is 'Bella's New Path' where she discovers new powers within the Volturi. It has an interesting take on her relationships with the Volturi members.
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 the Twilight universe where this fanfiction is based, the Volturi kings are Aro, Caius, and Marcus. They are the leaders of the Volturi, an ancient and powerful coven of vampires.
Maybe Edward joins the Volturi to gain more power. He might see it as a way to protect Bella in a different, more forceful way. The Volturi are very powerful, and he could learn new skills or gain access to resources that would help him keep Bella safe from other threats in the vampire world.
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.
There could be other vampires that Bella meets during her time as a vampire before the Volturi. For example, some lone vampires who are also trying to stay out of Volturi's radar. They could become her allies or foes depending on their motives.