Bella and her sister are the key ones. But depending on the story, there could be the sister's partner, if he's in the picture. Friends of Bella or her sister could also be significant characters. They might offer different perspectives on the pregnancy, like giving advice or sharing their own experiences.
The main character is surely the sister related to Bella. But in a fanfiction about 'imprint', there could be a whole host of other characters. If it's set in a supernatural world, there could be other supernaturally - inclined characters like witches or vampires who interact with the sister because of her imprint. Or perhaps there are human friends who are trying to understand what's going on with her. And of course, if the imprint has a romantic aspect, the person she has an imprint on would be a crucial main character as well.
It could be about the sister's journey through pregnancy, like dealing with morning sickness, doctor visits, and preparing for the baby. Maybe there are family reactions to the news as well.
The main characters are clearly Bella and Quil. Bella is likely to be the one carrying the baby, and Quil is the father - or at least an important figure in the pregnancy storyline.
The main characters are Embry and Bella. Embry is likely a werewolf in this fanfiction context, and Bella is the one who is pregnant.
The main characters are Bella and Aro. Bella is often a human - turned - vampire or has some special connection to the vampire world. Aro is a powerful vampire with his own set of abilities and a complex personality.
There could also be new characters introduced in the fanfiction. Maybe a doctor who specializes in the unique situation of a human - werewolf pregnancy, or a friend who has been through a similar experience and can offer advice to Jacob and Bella. But overall, Jacob and Bella remain at the heart of the story.
Bella herself could also be a significant character in this fanfiction. Her relationship with Jacob and her sister would be crucial. Her reactions to any potential relationship between Jacob and her sister could drive the story forward. Also, other characters from the Twilight series like Edward might make appearances, especially if they are involved in the overarching story related to Bella.
The bella sisters are the primary characters. In addition to them, there might be a wise old mentor figure who guides them in their adventures or challenges. There could also be a rival character who creates obstacles for the sisters, making the story more interesting and full of tension.
Well, in addition to Bella and Emmett, there could be other characters from the original source material (assuming it's based on something like Twilight series). For example, Bella's friends might play a role, like they could be there for support or they might be shocked and have to deal with their own feelings about the situation. Emmett's family or vampire clan (if relevant) could also be important characters, as they might have their own reactions to the pregnancy.
The main characters are obviously Bella and Edward. Bella is the human who is pregnant, and Edward is the vampire who is the father of the baby.