Then there are those like Izuku Midoriya. He might study the fanfiction from a different perspective. Since he's always analyzing quirks and hero strategies, he could look at how the authors in fanfiction have reimagined the characters' quirks and relationships. He might even get some inspiration from it for his own hero analysis notebooks.
The heroes in MHA like All Might might view Dabi fanfiction with a sense of concern. All Might has always been about justice and the greater good. He might see Dabi fanfictions as a sign of how some people are attracted to the darker side of the hero - villain dynamic. He could worry that it might influence younger fans in the wrong way. However, villains in MHA might have different reactions. Shigaraki could see Dabi fanfictions as a way to understand how the public perceives Dabi and use that knowledge to his advantage in his evil plans.
Some characters like Tsuyu would be quite level - headed in their reactions. She might simply take the fanfiction as an interesting take on the War Arc. If it has some inaccuracies, she might just shrug it off as creative license. But for characters like Mineta, he could be all over the place. He might be excited if he's shown as more heroic than he actually was in the War Arc, or he could be sulking if he's made to look like a fool.
For the teachers in My Hero Academia, like Aizawa, he would probably be indifferent to most ships fanfiction. But if there were ships that involved his students in inappropriate or unethical ways in the fanfiction, he would be very concerned. Otherwise, he would just see it as another strange aspect of the fans' creativity and leave it at that.
Well, some characters might be shocked. For example, Deku would probably be very concerned about how he's portrayed in the fanfiction. He might be a bit embarrassed if it shows his more vulnerable moments during the War Arc. Bakugo, on the other hand, could be angry if the fanfiction doesn't show his strength accurately. He's all about being seen as powerful, so any misrepresentation might tick him off.
Well, some of the characters might be really excited. Deku, for example, would probably be super interested in the different powers and hero concepts in Marvel fanfiction. He might take notes on how Marvel heroes use their powers and try to find ways to apply similar ideas to his own hero journey.
In a fanfiction that gives them new powers or abilities, most of class 1a would be intrigued. Sero might be immediately thinking of how to use those new powers in combat. However, if the new powers are too outlandish or don't fit their characters at all, there could be some pushback. Jiro might be critical and say that it doesn't make sense for her character to have such a random power that has no connection to her music - based quirk.
Some characters could actually be flattered. They may enjoy seeing how much the fans care about them and their story. However, there could also be characters who are confused by some of the more out - there concepts in fanfiction, like when a character from a different universe is introduced into the My Hero Academia world in a fan - made story. They might wonder how such a thing could be possible within the context of their world.
On the other hand, Bakugo might be enraged. He's a prideful character and if the fanfiction misrepresents him or shows him in a way he doesn't like, he could get really angry. He might even go on a rant about how inaccurate it is compared to his real self and his ideals of heroism. Also, if the fanfiction portrays him as weak, that would definitely set him off.
They could also be offended. If the fanfiction portrays them in a way that goes against their values or character traits in the original work, they may feel misrepresented. For example, if a character is always shown as noble in the original but is made into a villain in the fanfiction. They might not like this change at all and feel insulted.
They might be amused. Since fanfiction often contains creative and sometimes humorous takes on the original story, characters could find it funny to see how fans imagine different scenarios for them.