There could be many. For example, some fanfictions might focus on the backstories of the characters that were not fully explored in the original. Maybe a story about how one of the characters got into their fighting style before coming to the school.
Gray is often a popular one. His cool and mysterious demeanor makes him a great focus for fanfiction writers. They can explore more about his past traumas and how he overcomes them.
In the 'weak hero novel', the story might revolve around a not - so - physically - strong hero. This hero could be an underdog in a world full of stronger or more intimidating characters. He may use his wits, like devising clever strategies to deal with the problems. For example, when facing bullies, instead of using brute force, he might outsmart them with his knowledge or unique skills, which makes the story really interesting.
I'm not sure exactly as there could be various main characters depending on the specific 'weak hero novel'. But generally, there would be the weak hero himself, perhaps a bully or two who act as antagonists, and maybe some friends or allies that the weak hero makes along the way.
Typically, 'fff class hero' novels often feature a protagonist with rather unusual powers or skills. These skills might not seem very impressive at first glance, like being able to do something very mundane but in an extraordinary way.
The hero might be defined by their moral code and personal values rather than their social class. They could rely on their wit, courage, and kindness to overcome challenges.
I'm not entirely sure as I haven't fully read it. But generally, it might involve a hero who has faced some sort of 'disaster' or great challenge before and is now making a comeback. It could be about his journey to regain his former glory, face old enemies, and perhaps also deal with new threats that have emerged in his absence.