Bella's mother or a close female friend could also be main characters. Her mother could share her own experiences of giving birth and offer practical advice. The close female friend could be there to hold Bella's hand, offer comfort, and maybe even crack a few jokes to lighten the mood during the intense moments of the birth.
Her partner could be a main character. They would be deeply affected by her death and would likely have to take on a lot of responsibilities, especially in terms of raising the baby. The baby itself could also be considered a main character in a sense, as the story could follow its growth and development in the absence of its mother.
Well, it could be about the challenges Bella faces during the birth, like the pain and fear. Maybe it also shows the support she gets from her family or friends around her.
The baby, of course. Growing up without a mother would be a huge part of the story. The father would also be deeply affected, having to raise the child alone and deal with the grief of losing his partner.
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.
The Cullen family usually plays a significant role. Carlisle, with his medical knowledge (even if it's vampire - related in the Twilight context), might be involved in ensuring a safe birth. Alice could be there, perhaps using her gift to foresee any potential problems during the birth. Even the other members of the Cullen family would be around to offer their support in different ways.
The truck itself could be considered a main character in a sense. It has its own presence and significance in the story.
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.
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.
If it's a more modern - day or non - supernatural setting, Bella's parents could be main characters. They might be overjoyed at the birth of their grandchildren and be actively involved in their upbringing. They could provide financial support or just be there for emotional support for Bella, especially during the tough early days of raising twins.
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.