It depends on the specific situation and the character's personality. Some heroes might have moments of fear but overcome them through their determination and sense of duty.
A villain like Shigaraki would likely be interested in the power levels in the MCU fanfiction. He's always looking for ways to increase his own power, and seeing the various powerful artifacts and abilities in the MCU might give him some ideas. He could think about how he could use similar things in his own plans to take over the MHA world.
It could be about a character in the My Hero Academia universe who defies the traditional concept of a hero. Maybe they don't have the typical flashy quirk but still manage to be heroic in their own way, like using intelligence or empathy.
Tadashi himself, without his memories, would be confused. He'd see all these people who seem to know him well but he doesn't recognize them. He might feel a bit scared and out of place. As he starts to interact with the others more, he'd slowly form new relationships with them while they're trying to bring back the old ones. There could be some really interesting character dynamics as everyone adjusts to this new situation.
Characters could have mixed reactions. If the fanfic is well - written and respectful of the source material, they might be quite positive. But if it's poorly written or changes their characters too much, they could be disappointed or even angry. For example, if a character is known for being brave in the movie but is made cowardly in the fanfic, that character would likely be unhappy about it. And this reaction could vary from character to character depending on their personalities and how much they value their established identities.
Hiro would be determined to save the kidnapped member. He'd immediately start analyzing the situation, using his intelligence to figure out where they might be held. Baymax would be concerned for the safety of the kidnapped character and would follow Hiro's lead. GoGo would be angry and ready to take action, using her speed to quickly track down any leads. Wasabi would be a bit more cautious but still committed to the rescue. Fred would probably be coming up with wild but potentially useful ideas on how to save their friend.
Some MHA characters might be flattered. For example, Midoriya would probably be really excited and take it as a sign of how much he's inspired people. He might even look for positive lessons in the fanfiction to improve himself.
If the MHA characters were to react to a Mastermind fanfiction, some would be more accepting than others. Characters like Ochaco might find it really exciting. She's a fan of the heroes herself, so seeing different takes in fanfiction would be like getting a new perspective on the MHA world. However, characters like Shoto, who are more reserved, might be a bit hesitant. He might need some time to process what he's reading. But overall, it would be a very interesting scenario as it would show how the characters view different interpretations of their world.
Some characters would be shocked. For example, if the fanfiction had a really dark and unexpected Mastermind plot, like a hero turning out to be the mastermind of all the villains' plans, characters like Tsuyu would be very taken aback. She'd probably question the motives behind such a plot. Then there are characters like Denki. He might be more into it if it had some cool action scenes in the fanfiction, regardless of how out - of - the - blue the Mastermind storyline was. And Nejire would probably be super curious about the emotional depth of the characters in the fanfiction.
If it's a Disney fanfiction based on a princess story, the female characters in MHA like Momo Yaoyorozu might relate to the princesses' grace and leadership. Meanwhile, the male characters could either be in awe or a bit dismissive, depending on their personalities. For example, Todoroki might be quietly respectful, while Mineta might make some inappropriate remarks (if we're being honest about his character).