Obviously, Bella and Sam are the main characters. Bella is the one who is pregnant, so she will likely be dealing with a lot of emotions and physical changes throughout the story.
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.
The main characters are obviously Sam, Jessica and their baby. Sam and Jessica are the parents, and the baby is the new addition to their family that will likely drive the story forward.
The main characters are clearly Johnny and Baby. They are likely the focus of the story as they navigate through the situation of the pregnancy.
Of course, the baby is also an important character in a way. Although it may not be able to communicate much at first, it will influence the actions and decisions of Bella and Jacob. Their families and friends, like the Cullens or the other members of Jacob's pack, could also be considered secondary characters as they will interact with Bella, Jacob and the baby in different ways.
Another possibility is that there is a mysterious figure who has some sort of vendetta against the family. This figure could be causing strange and unexplained events to occur around the baby, like making the baby sick in a way that doctors can't figure out at first. This would then be a source of conflict as Bella and Sam try to protect their baby from this unknown threat.
Obviously, Sam and Bella are the main characters. Sam is often depicted as the hero or the savior. He might be portrayed with his own set of powers or just his strong will and physical abilities. Bella is the one in need of saving, and her character is usually based on the original Bella from the Twilight series, with her own insecurities and kind heart.
There could also be some human characters in the fanfiction who are friends with Bella. These characters might not understand her relationship with Sam at first and could cause some drama. They might try to pull Bella back into the normal human world, while she's being pulled towards Sam and the werewolf world.
It could be demons. Demons are often the main antagonists in the Supernatural universe, and they might target Bella because she is related to the Winchesters. They could try to use her as a bargaining chip or a way to get to the brothers.
The main characters are clearly Emilia and Maou. Emilia is often depicted as a strong - willed and kind - hearted character, and in this context, she is the one who is pregnant. Maou, on the other hand, is likely to have his own set of characteristics, perhaps being more powerful and having a complex personality as he is often associated with a more 'demonic' nature.
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.