Without more details, Kenny could be anyone. He might be the protagonist of the story, a person with his own motives and desires. Maybe he's a young man who gets involved in a mystery and the secret is central to his journey. It's also possible that he's a more complex character with a backstory that led him to the point of abandoning this secret.
Mrs Black could be so offended that she starts to plot against Hermione. For example, she might try to get Hermione into trouble with the Hogwarts authorities by spreading false rumors about her.
Harry might become more determined to prove Snape wrong. He's a stubborn character, and Snape's yelling could fuel his desire to show that he's not the misbehaving student Snape seems to think he is.
Perhaps in the fanfic's storyline, Tony is facing a lot of pressure himself, like threats from powerful villains or issues within the Avengers. Peter's actions, whether they are mistakes or just inconvenient timing, could be the straw that breaks the camel's back for Tony, causing him to yell at Peter.
Perhaps Ruby made a decision that Weiss strongly disagreed with regarding a mission objective. Ruby thought it was a good idea to help a group of civilians first, but Weiss believed they should focus on the main target. Weiss would yell, 'Ruby, you're being naive! We can't deviate from our mission just for some strangers!' This would start a heated debate between them, with the other team members having to step in to calm them down.
It's hard to say. 'Levi x Eren' fanfics are quite common in certain fandoms, but the 'Levi is a criminal' aspect makes it a more specific and perhaps niche sub - genre. If there are a lot of fans who like to see dark or morally ambiguous takes on the characters, it could potentially be popular within that subset of the fandom. However, it also might not be mainstream popular as it goes against the more typical heroic portrayals of the characters.