Well, Sonic is obviously a main character since the whole concept is about him getting framed. Tails is often another main character. He could be initially misled into believing Sonic is guilty but then start to have doubts and help Sonic prove his innocence.
Another character that could be a main character is Dr. Eggman. He might be the one behind the framing in many fanfictions. He wants to get rid of Sonic once and for all and uses the framing as a way to turn Sonic's friends against him so that he can finally conquer the world without Sonic interfering.
Obviously, Sonic and Metal Sonic are the main characters. Sonic is the one being framed, and Metal Sonic is the one doing the framing.
I'm not sure specifically as there could be various interpretations in such a fanfiction. Maybe Metal Sonic sets Sonic up in a way that makes Sonic look bad to the other characters, perhaps by faking some evidence or causing chaos and making it seem like Sonic's fault.
Well, in some Sonic fanfiction, the idea of Sonic getting framed by Metal Sonic can involve a complex plot. Maybe Metal Sonic sets up Sonic to look like he's done something wrong, like stealing a chaos emerald or causing chaos in the city. It could be part of a villain's scheme to turn Sonic's friends against him.
One possible plot could be Sonic being framed for stealing the Chaos Emeralds. He then has to go on a journey to prove his innocence while facing his friends who have turned against him.
Often, it begins with Metal Sonic being given some sort of motivation by an outside force or just his own envy. He then starts to plan how to frame Sonic. He might first create a false identity or a false trail that leads back to Sonic. This could be as simple as leaving Sonic - like items at the scene of a 'crime' he's supposed to have committed.
In some Sonic fanfics, it could be a rival character seeking to cause trouble for Roger Rabbit.
Sonic himself, of course. And his best friend Tails is very likely to be there. Tails is always by Sonic's side and would surely be there when Sonic is sick.
Sasuke could be a likely character in some fanfiction. Maybe in an alternate universe where their rivalry takes a darker turn, Sasuke might frame Naruto to gain an advantage or to prove his own superiority. It would add a whole new dimension to their relationship.
Dr. Eggman would be a likely main villain. He might be the one who devises a plan to make Sonic fat, thinking that it will slow him down and make it easier for him to be defeated.
Another possibility is a corrupted version of one of Sonic's friends. Maybe through some dark influence, like an evil Chaos Emerald, a friend turns on Sonic and blinds him. This would add an extra layer of drama as Sonic has to deal not only with his blindness but also the betrayal of someone he trusted.