Bakugou might react with his usual anger. He could lash out at the bullies verbally or even physically, which might make the situation worse. For example, if someone makes a snide comment, he could respond with a loud and aggressive retort.
If it's a fanfiction about Class 1A meeting Bakugou's boyfriend, there could be a lot of comical moments. Maybe Mineta would try to be inappropriate and get scolded by Bakugou. The more responsible students like Iida might give Bakugou relationship advice. His boyfriend could be from another class or even another school. If he's from another school, Class 1A would be curious about the differences in their hero training. And Bakugou would have to balance his relationship with his training and his place in Class 1A.
One possible plotline is that Bakugou's boyfriend is initially a mystery to Class 1A. They start to notice Bakugou acting differently, like being less angry or more distracted. When they finally meet the boyfriend, he could be someone unexpected, like a shy artist type. This would contrast with Bakugou's bold personality. Another plotline could be that the boyfriend is a powerful hero - in - training from another school. Class 1A might be a bit jealous at first but then realize they can learn from him. And there could be a plotline where the relationship is a secret at first and Class 1A accidentally discovers it.
Bakugou might be initially shocked. He's used to Izuku calling him 'kacchan' for so long that it's become a normal part of their relationship. So, when Izuku stops, it could throw him off balance.
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.
He should listen attentively. Look into her eyes, nod his head to show he's following along, and make small comments like 'Really?' or 'That's amazing'.
Some characters might try to understand Deku's motives. Kirishima, who was close to Bakugou, would be angry at first but might also try to find out if there were any extenuating circumstances. However, many would be afraid of Deku, seeing him as a threat now that he has killed one of their own.
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.