It could be about a fan - created story in the My Hero Academia (MHA) universe where Bakudeku (presumably a relationship or dynamic between Bakugo and Deku) are involved with a secretary agency. Maybe they work there, or the agency has something to do with their hero activities.
You can try looking on fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. They have a large collection of My Hero Academia fanfics, and you might be able to find the 'bakudeku secretary agency mha fanfic' there.
One common theme is the power dynamic. Fat Gum has a lot of strength and experience, while Fem Kirishima might be portrayed as more of a newbie or someone looking to learn from him. Their relationship can develop around training and growing stronger together.
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.
You can start by looking on fanfic websites like Archive of Our Own (AO3). It has a large collection of My Hero Academia fanfics. Just use the search function and type in 'MHA Fem Kirishima x Fat Gum' and you'll likely find a bunch of relevant stories.
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.
They'd be surprised. The more innocent ones like Tsuyu might just blink in confusion. And Todoroki might raise an eyebrow, not fully understanding what it's all about.
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).