There could be a new character introduced. This character could be someone from Bella's past or a completely new entity. Maybe this character has a claim on the person Bella loves, and Bella, being the selfless person she is in this fanfiction, steps aside to let this new character have a chance at love.
There could be Bella's partner in the relationship she is betraying. This character might be a loving and trusting person, which would make Bella's affair even more complicated and morally wrong.
Well, apart from Bella, there could be a male lead who is the creative director of the 'for him' brand. He could be a strict and demanding professional but also has a soft side. Then there are the makeup artists and hair stylists who are always around Bella during shoots. Also, there could be Bella's agent who is constantly trying to get her the best deals and manage her career. There might be some models from other agencies who are jealous of Bella's rise in the 'for him' modeling world.
There could be her friends. For example, Alice might be there. Alice could be trying to calm Bella down during her tantrum, using her calm and composed nature to soothe Bella's anger.
One idea could be that Bella steps aside because she realizes her love is causing more harm than good to the people around her. Maybe she sees that her presence is putting her friends in danger, so she decides to leave for their safety.
A potential main character could be Bella's best friend. This friend could provide support and a different perspective as Bella navigates her new identity. There might also be a mentor figure for Bella, someone who knows a lot about Elijah's world and can guide her. This could be an old friend of Elijah's or a wise elder in their community.
The truck itself could be considered a main character in a sense. It has its own presence and significance in the story.
She could interact with Elena. Since Elena is a central character in Mystic Falls, their paths are likely to cross. They might start as friends or rivals, depending on the story.
There could be Edward. Even though the focus is on Carlisle and Bella, Edward's presence might still be felt as he was an important part of Bella's life before. His reactions to Carlisle and Bella's relationship could add some drama.
Often, you'll see Edward in these fanfictions. Even though the story is about Bella as a vampire before Twilight, Edward still plays a role. Maybe he's a mystery to Bella at first, or he's from a different vampire group that she has to interact with. Another common character is Jacob. He could be in a more antagonistic role, being jealous of Bella's new vampire life. And of course, the Cullens might be mentioned or even play a part. Carlisle could be a figure that Bella looks up to for advice on being a 'good' vampire.
Perhaps a new character could be introduced as the hero. A wizard or a powerful human with some extraordinary skills. This new character could be someone who has been observing Peter's actions from afar and decides to step in to save Bella when the time is right.