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.
Sometimes, in this type of fanfiction, there could be original characters created by the author. These characters could be added to bring in new storylines or to serve as catalysts for the events related to Gaster's pregnancy. They might be doctors or nurses if the story delves into the medical aspects of the pregnancy, or they could be mysterious figures with their own agendas regarding Gaster and his unborn.
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.
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.
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, Daenerys herself is the central character. But often, characters like Tyrion Lannister might be involved. He could be her advisor during her pregnancy, providing his sharp political insights. Jon Snow could also be a part of it, perhaps in a more complex relationship with Daenerys given their previous interactions in the original story. And of course, her loyal followers like Missandei and Grey Worm would likely be by her side, protecting her and the unborn child.
Often, other Decepticons like Starscream might be involved. Starscream, always vying for power, might see Megatron's condition as an opportunity to finally take over. So he could play a major role in the fanfiction, either causing trouble or having a change of heart. And then there could be some human characters who get caught up in the events, perhaps scientists or soldiers who are trying to understand what's going on with Megatron.
Max herself could be a main character in such fanfics. Since she's the central figure in the 'Maximum Ride' series, it's likely that a story about pregnancy would revolve around her.
Obviously, Asuna and Kirito are the main characters. Asuna being pregnant, Kirito would be by her side, protecting her and preparing for the new arrival. Their relationship would be the central focus, showing how they support each other through this new phase.
Alina is also frequently involved. Her connection to the Darkling in the original work makes her a natural addition in many fanfictions. Their complex relationship can be further explored or even rewritten in different ways in fanfiction.
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.
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.