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.
Mainly, Tails is the focus. However, characters like Cream the Rabbit and Cheese could be in the story. Cream might be curious about Tails' new appearance due to the weight gain and Cheese could add some comical elements. Big the Cat could also be involved. He might have some advice for Tails based on his own laid - back lifestyle, even if it's not the most helpful advice.
Tails is the central figure in 'tails weight gain fanfiction'. His friends like Amy Rose could also be involved. Amy might be worried about Tails' sudden weight gain and try to help him with a diet or exercise plan. Knuckles could also be part of the story. He might not understand at first what's going on with Tails but would be ready to protect him if there was any danger related to his weight gain situation.
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.
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.
The Jinchuriki of the Two Tails, Yugito Nii, is often a main character. She has a complex relationship with the tailed beast inside her.
Naruto himself is often a main character. Since he has a strong connection to the tailed beasts in the original series, in fanfiction involving the Ten Tails, he is usually at the center of the action, trying to either defeat it or understand it better.
Naruto is often still a main character in such fanfiction. Since he has a strong connection with the tailed beasts already. Sasuke might also be involved as he is another important figure in the Naruto universe. And of course, the new hosts of the reincarnated tailed beasts would be main characters as well.
In a lot of 'zero tails naruto fanfiction', Naruto is front and center. He has a knack for getting involved in extraordinary situations. Kakashi could also be a main character in such fanfictions. His experience and wisdom could be crucial in understanding and combating the zero tails. Additionally, characters like Shikamaru, with his intelligence, might be involved in devising strategies to handle the new threat posed by the zero tails.
Often, Sonic characters like Sonic himself or Tails might be involved. Since the Tails Doll is related to the Sonic franchise, they could be trying to save their friend or the world from the curse. Maybe Sonic, with his speed and courage, is on a mission to stop the Tails Doll's curse from spreading further.
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.
Sonic is often paired with Tails. Their friendship in other Sonic canons carries over, and in Sonic Underground fanfiction, they might have new adventures together. Sonic's speed and Tails' smarts make them a great team.