Loki and his wife are the primary characters. But we can't forget about the servants or attendants in Loki's household. They could be involved in taking care of Loki's wife during the pregnancy. Additionally, there could be some magical creatures that are native to Asgard or other realms that play a part in the fanfiction, maybe they are there to bestow blessings or cause some sort of magical phenomenon related to the pregnancy.
Loki is the main one for sure. His wife, of course. And then there could be other Asgardian characters like Heimdall. Heimdall, with his all - seeing abilities, could play a role in protecting Loki's wife during her pregnancy. There might also be some of Loki's old allies or foes from his past adventures who get involved in the story, either to help or cause trouble.
Jaune could be a popular character in such fanfictions. If Ruby is in a relationship with him, he would play a major role in the story. He could be shown as being nervous yet excited about becoming a father. Pyrrha might also be involved in some way, perhaps in flashbacks or as a guiding spirit for Jaune, even though she's no longer alive in the main series.
Obviously Clary herself is the central character. Jace is also very often involved as he is Clary's love interest. Alec and Isabelle Lightwood might also play important roles. They could be the ones helping Clary through her pregnancy, giving her advice or protecting her from any threats.
Mal herself is likely to be a very popular character in such fanfiction, as she is a central figure in the 'Descendants' world. Her story arc can be easily adapted to include a pregnancy plot. Evie might also be involved, either as a friend supporting Mal or having her own related story. Jay could be another character who might play a role, perhaps as a protector or someone with a connection to the events related to the pregnancy.
Well, obviously Gaster is the central character as it's about his pregnancy. But often, other characters from the same fictional world might be involved. For instance, characters who are friends or colleagues of Gaster could be there to support or oppose the situation. It might also include characters who have some sort of special connection to Gaster's unborn child, like a potential mentor or an enemy who wants to harm the baby.
Rias Gremory is often a popular character in such fanfics. Given her central role in the original Highschool DxD series, her relationship with Issei could be explored in the context of a pregnancy. Her status as a devil and the leader of her peerage would add a lot of complexity to the story if she were to become pregnant.
Kristoff is often involved. He would be there to support Anna, and might be a bit nervous about becoming a father. Olaf could also be a popular character in such fanfictions. He'd be his usual cheery self, trying to make the pregnant Anna and Elsa happy with his funny remarks.
Obviously, Bella Swan and Edward Cullen are the main characters. Since it's about Bella being pregnant, they are at the center of the story.
Gale Hawthorne might also be involved. Since he has a complex relationship with Katniss in the original story, his reaction to Katniss's pregnancy could be an important part of the fanfiction. He might have feelings of jealousy or regret, or he could be a supportive friend in a different way.
Obviously, Quinn is a main character as she is the one who is pregnant. Finn might also be heavily involved as he was Quinn's boyfriend in the show and could play different roles in the fanfiction, like being a supportive partner or facing difficulties in dealing with the pregnancy.
Edward and Bella from 'Twilight' are often used in difficult love fanfiction. Their relationship has elements of danger and the struggle between Edward's vampire nature and Bella's human existence. Another pair could be Harry Potter and Hermione Granger. Some fanfictions explore a more romantic relationship between them, which is difficult considering their strong friendship and the overall plot of the original series.