Some characters might be flattered. For example, Izuku Midoriya would probably be really shy but also a bit happy that people are so interested in his story and his growth as a hero. He might see it as a sign that he's making an impact.
Characters like Bakugo Katsuki might react with anger or annoyance. He's prideful and might not like the idea of people writing fictional stories about him without his permission. He could see it as a form of disrespect or people making assumptions about him. On the other hand, Ochaco Uraraka might be more open - minded and find it funny or interesting. She has a friendly and approachable personality, so she could potentially enjoy reading some of the more positive fanfiction about herself and her friends.
They may be either amused or annoyed. It depends on how they are portrayed in the fanfiction. If it's a positive and creative portrayal, they might be amused. But if it's a negative or inaccurate one, they could be annoyed.
There are those who would be critical. Characters like Iida might analyze the fanfic for its accuracy in terms of the My Hero Academia universe's rules and ethics. If there are any inaccuracies, he might be quick to point them out.
Iida would probably be very vocal about any inappropriate or unethical content in the fanfiction. He has a strong sense of justice and propriety, so if there are any elements that go against his values, he would not hold back in expressing his disapproval.
Shigaraki would likely be disgusted if the fanfic made him seem weak or less of a threat. He'd probably scratch at his neck in irritation. Dabi would be dismissive. He'd think fanfics are just a waste of time and not worthy of his attention. Toga, though, might be intrigued. If the fanfic had a lot of blood and gore in it, or if it focused on her relationships in an interesting way, she could be really into it.
The heroes in MHA would likely be initially shocked and a bit intimidated by the Doom Slayer. His brutal and direct approach to combat is very different from their more heroic and often rule - bound ways. Bakugo might be one of the first to be both intrigued and challenged by the Slayer's presence, as he respects power. The villains, on the other hand, would probably see the Doom Slayer as a major threat and might try to avoid direct confrontation at first.
The female characters like Ochaco and Tsuyu might feel a sense of relief. Ochaco has often been on the receiving end of Mineta's inappropriate behavior, so she would probably be glad that she no longer has to deal with that. Tsuyu, being the no - nonsense type, would likely think it was a long - overdue consequence.
They would probably have a range of reactions. Some might be excited to see a different take on heroism. Others might be skeptical. For instance, Todoroki could be interested in how the characters in One Punch Man deal with their powers compared to how he deals with his quirk.
His friends would be really shocked at first. Uraraka might be the first to try and comfort him, seeing as she has a kind heart. Bakugo, on the other hand, would probably be both confused and a bit angry at the situation. All - Might would likely feel a sense of responsibility to protect Deku even more than before. The teachers at UA would probably start researching how to reverse the transformation immediately.
Todoroki would be rather indifferent on the surface. He'd read it with his usual calm expression. But if the fanfiction delved into Deku's relationships with his friends, Todoroki might start to think more about his own friendships. If there was a part where Deku helped someone in a really selfless way, Todoroki might respect that and see it as something to strive for in his own actions.
Shigaraki would likely be dismissive. He's so focused on his own goals of destroying the hero society that he'd see fanfiction as a useless distraction. However, if there were elements in the Genshin Impact fanfic that could somehow be related to his own plans, like powerful artifacts or magic that could be used for destruction, he might take a second look.