Quinn Fabray could be involved too. Given her history of being concerned about her looks and status, a weight gain story for her could be about her learning to love herself regardless of the number on the scale. It could also involve her relationship with the other Glee club members as they support her through the process.
Well, obviously Tails is the main character as it's about his weight gain. But Sonic often shows up in these fanfictions too. Sonic might be there to support Tails, or he could be initially confused about what's happening to his friend. Another character could be Dr. Eggman. He might be the cause of Tails' weight gain, either intentionally through some evil plan or accidentally because one of his inventions backfires.
Twilight Sparkle is often involved. As a central character in the My Little Pony universe, her studious and sometimes self - conscious nature can be interesting to explore in the context of weight gain. Pinkie Pie could also be a main character as her love for parties and food might lead to some scenarios of weight gain. And Rainbow Dash, with her athletic build, could have an interesting character arc if she were to experience weight gain, perhaps struggling with her identity as an athlete.
His Pokémon are central, as they are always by his side. Pikachu would be a key figure in any story related to Ash. Then there are his human friends. Brock could be involved in cooking for Ash, which might contribute to the weight gain. Misty could be there to comment on Ash's appearance and give her opinions. And in some fanfictions, Nurse Joy or Professor Oak might also be included to provide medical or wise advice regarding Ash's situation.
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.
Finn is often a main character in such fanfictions. Since he is the protagonist in the original 'Adventure Time', it's natural for writers to explore different scenarios involving him, like unexpected weight gain and how he copes with it.
Kurt Hummel could be another. He is known for breaking gender norms in the show. A male pregnancy plot for him could be a way to further explore his character's journey and how he navigates such an extraordinary situation, as well as how his friends and family react.
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.
Homer could be involved. Since Homer loves food himself, he might either encourage Bart's new eating habits that lead to weight gain, or he could be the one trying to help Bart get back in shape. Marge would also be a key character. She would likely be concerned about Bart's weight gain and try to get him to eat healthier, constantly nagging him to stop eating junk. Lisa could also play a role. She might be initially teasing Bart about his weight gain but then feel bad and try to find a scientific way to help him lose the extra pounds.
Well, Cersei herself is obviously a main character. Since it's a fanfic about her weight gain, the story would revolve around her experiences, emotions, and how she deals with this change.
I don't know since I don't have the details of the 'weight gain game story'. It could be a character named Bob who is trying to gain weight for a wrestling match in the game.