Perhaps some of the other Cullen family members would be her allies. They are already vampires and could help her adjust to her new powers and the vampire lifestyle. For instance, Alice might use her power of seeing the future to guide Bella in making important decisions. Jasper could teach her how to control her new vampire instincts, like the urge to hunt. And Emmett could be her sparring partner to help her improve her combat skills.
In 'bella becomes a badass vampire fanfiction', she could develop unique vampire powers. For example, she might be able to control the minds of other vampires. This would put her in a powerful position within the vampire world. She could also use her new status to change the way vampires interact with humans, making it more peaceful or something like that.
There's also a possibility that some of the more morally grey characters in Gotham, like Catwoman, could be Bella's allies. Catwoman has her own code of ethics, and she might relate to Bella's struggle between good and evil. They could work together in certain situations, especially if there are mutual benefits involved.
Edward could be one of her allies. He could use his vampire abilities such as super - speed and strength to assist Bella in dangerous situations. His centuries - old knowledge could also prove invaluable in understanding the motives and actions of the CIA's enemies. Another ally could be Jacob. His shape - shifting abilities would give the team an extra edge in combat or in covert operations. He could also use his connection to the werewolf pack to gather information from areas that the CIA might not have easy access to.
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.
Edward would still be a very important character. Since he's Bella's love interest in the original 'Twilight' series, in the fanfiction, he would play a major role in Bella's new vampire life. He might help her adjust to her new powers and the vampire lifestyle.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of Twilight - related fanfictions, and you're likely to find some great 'badass vampire bella fanfiction' there. Just use the search bar and type in relevant keywords.
One popular 'badass vampire bella fanfiction' might be those where Bella has enhanced combat skills. For example, in some stories she is trained by ancient vampires to become a formidable fighter, using her newfound abilities to protect the Cullen family instead of always being the one in need of protection.
Her parents might be important characters. They could either be supportive of her career choice or initially be against it, thinking it's too difficult. Also, there could be a senior lawyer at the firm where she interns, who takes her under his or her wing and guides her through the complex world of law.
Obviously, Bella is one of the main characters since it's about her being a badass. There might also be other characters from the original story she is from, like Edward or Jacob, who could be involved in the new storylines around badass Bella.
Obviously, Bella is a main character as the title implies she is the one becoming a vampire. The Cullen family members are also likely to be main characters. Edward, who was in a relationship with Bella when she was human, will surely play an important role. Also, Carlisle, the patriarch of the Cullen family, might be involved in guiding Bella through her new vampire existence.