Well, Sirius would have a complex reaction. On one hand, he was very close to James, and his brother was Regulus. The idea of them in a romantic relationship in fanfiction would be hard for him to wrap his head around. He might start to question how people could come up with such a thing. He might also be a bit protective of James' memory, if he thinks the fanfiction is portraying James in a way he doesn't like. However, he might also be secretly a bit intrigued, because it's such a different take on the relationships he knew, and it might make him think about James and Regulus in a new light, but he'd never admit it out loud.
He would probably be furious. The blood quill was a very cruel and unjust tool used on him as a student. Discovering fanfiction about it might make him feel that his past pain is being exploited or made light of.
Sirius could be outraged. He might feel that it's inappropriate to create fictional relationships between his best friend and his brother. He could even try to find out who is writing it and ask them to stop. However, he also might realize that he can't really control what others write in their free time.
In the fanfiction world, Snape might be initially shocked. He has a complex relationship with Harry. He could feel a sense of protectiveness that he rarely shows openly. He might use his skills and knowledge to find the perpetrator and seek justice for Harry.
He would probably be very displeased. Snape has always been strict and traditional, and fanfiction about the DA, which was an unauthorized student group in his eyes, would seem like a frivolous and inappropriate thing to him.
Well, Percy has seen a lot in his adventures. At first, he might be a bit taken aback because fanfiction is a whole new world compared to his real experiences. However, as he realizes that it's just fans exploring different scenarios with the characters, he could become rather indifferent. He might even start to see the value in it, as it shows how much the fans care about the overall story and its characters. He might think it's a little strange that people are so interested in writing about Nico and Will's relationship, but he'd respect the creativity of the fans.
Harry would likely be quite curious. He'd want to know what these fanfictions were about. Maybe he'd be a bit cautious at the same time, worried that some of the fanfictions might distort Sirius's true character. But overall, I think he'd be intrigued and might even try to find a way to read some of them, just to see what the fuss was all about.
He might be quite shocked at first. After all, Wolfstar involves his dad's best friend Sirius and his other dad figure Lupin. It's a rather strange concept for him to think about them in a romantic way as he knew them in a different context.
She might be initially surprised. Fanfiction often presents alternate scenarios of relationships. Sakura has always had a complex relationship with Naruto, more of a deep friendship in the original story. If she found out about this in fanfiction, she could be flattered that someone thought of them in that way, but also might feel a bit awkward since their real - life relationship dynamic is different.
Well, Harry might start to look at Sirius in a new light. He could see him not just as his godfather, but as a figure that inspired creativity in others. This could lead to Harry wanting to learn more about Sirius's past, things that might not have been covered in the original stories he knew. And it could also lead to him having more conversations with people who wrote or read the fanfiction, getting new perspectives on Sirius.
Snape would be appalled. He has a deep connection to the world of Harry Potter, and the idea of a rape romance fanfiction is so far removed from the values of the wizarding world he knows. He might initially be in disbelief, thinking it was some sort of dark joke or a misunderstanding. But once he realizes it's real, he would likely feel a mix of anger towards the creator and sadness that such a thing exists in relation to a world he cares about.