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.
Well, the main characters are obviously Bella and Edward. Bella is the one who tries to make Edward jealous, usually by getting close to other male characters. Edward is the one who gets jealous because of his deep love for Bella.
While Bella and Edward are the main focus, sometimes the Volturi are mentioned in a more distant sense. Their presence in the overall Twilight world looms over the couple, but on the wedding night in fanfictions, it's more about Bella and Edward's personal and private moments. It's about their love, their dreams, and their new life together as husband and wife.
Obviously, Bella Parker is the central character. Often, her friends from the performing arts world, like her dance classmates or instructors, are also important characters. They might be there to support her, compete with her, or collaborate on various projects.
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.
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.
The main character is always Bella in this type of fanfiction. However, new characters could be introduced as well. For instance, a blind mentor who has learned to live fully with blindness could be added. This character could guide Bella and help her accept her condition. Or there could be a villain who tries to take advantage of Bella's blindness, which would then lead to a conflict where Bella has to use her wits and other strengths to overcome the villain.
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.
Often, there are midwives or healers that are introduced. These could be ancient vampires with special knowledge of hybrid births. Another common new character is a seer or prophet who can predict the future of Bella's baby. Sometimes, there are also new werewolves who come to protect Bella during the birth because they sense a great change is coming.
Often, there's a young wolf who is Bella's best friend in the pack. This wolf might be more impulsive and gets Bella into some fun but also dangerous situations. And there could be an older, more experienced wolf who acts as a mentor to Bella, teaching her about the ways of the pack.
Obviously, Bella is a main character. She is often the center of the story, dealing with issues related to her curvy body. Sometimes Edward might also be a main character, especially if the fanfiction is set in a Twilight - inspired universe and their relationship is explored in the context of Bella's body image.