Obviously, Gaster and the human are the main characters. Gaster is the one doing the adopting, and the human is the one being adopted.
Well, in 'gaster adopts a human fanfiction', it might start with the human character being in a world that is unfamiliar or dangerous. Gaster, who could be seen as a mysterious and powerful figure, notices the human. He might be intrigued by the human's nature or potential. So, he decides to adopt the human, which could lead to a series of events where the human has to adapt to Gaster's environment, learn about his ways, and perhaps they go on adventures together. It could also explore the relationship between the two, like how the human grows to trust Gaster and how Gaster comes to care for the human as a sort of parental or mentor - like figure.
Obviously, Gaster himself is likely to be a main character. His mysterious nature makes him a central focus as readers are eager to learn more about him. Another potential main character could be Frisk. Their ability to reset and influence the game's world could be very interesting when combined with Gaster's enigma. They could either be allies or have some sort of conflict due to their different understandings of the world.
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.
Obviously, Sans and Papyrus are the two main characters who we already know well from the original Undertale. They have distinct personalities that will play a big role in how they interact with Gaster. Gaster, on the other hand, is the enigma. His presence will likely drive the story as Sans and Papyrus try to figure out who he is now and what he wants, so he's also a main character in this fanfiction.
Obviously, Pepper and Peter are the main characters. Pepper, with her strong and capable personality, and Peter, the young and enthusiastic Spider - Man.
The main characters are Gibbs, who is a no - nonsense kind of guy from the NCIS world, and Harry. Harry is not just a normal kid but a wizard with a past full of battles against evil. There could also be other supporting characters from the NCIS team, like DiNozzo, who might be initially skeptical about Harry's magic but later become friends with him.
The main characters are Cedric and Harry. Cedric, a popular and talented student at Hogwarts, decides to take Harry under his wing. Harry, who has had a tough life so far, now has to adjust to living with Cedric. There could also be minor characters that are important for the story, such as Cedric's professors who might notice the change in Cedric's behavior due to his new responsibility of taking care of Harry, or Harry's enemies who might be more cautious now that Harry is under Cedric's protection.
Obviously, Kakashi and Sakura are the main characters. Kakashi, the experienced and skilled ninja, and Sakura, who is eager to grow and improve.
The main characters are obviously Percy and Nico. Percy, a well - known and powerful demigod, and Nico, who has his own unique powers and a rather troubled past.
The main characters are Simba and Kovu. Simba, as the leader of the pride, his actions of adoption set the story in motion. Kovu, on the other hand, is the character who will face new challenges, learn new things, and probably change as a result of being adopted. There may also be other supporting characters like Nala, who might have an opinion on Simba's decision, and the other members of the pride who will interact with Kovu.