Snape would likely be deeply offended. The blood quill is a symbol of his dark and difficult past at Hogwarts. If he knew there was fanfiction about it, he might see it as an intrusion into his personal and painful history. He's a very private person, and this would seem like a violation of that privacy.
Well, Snape is a complex character. If he found out about the blood quill fanfiction, he might first be in shock. Then his anger would start to build. He has always carried the scars, both physical and emotional, from the use of the blood quill. Fanfiction about it could reopen those old wounds. He might also feel a sense of betrayal, as if the wizarding world was once again turning his suffering into some form of entertainment, rather than respecting the trauma he endured.
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.
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, 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.
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.
In some fanfictions, Snape might be secretly sympathetic. He may see the scars as a reminder of all Harry has been through, which could soften his usually harsh attitude towards Harry, at least for a moment.
Snape would likely have a very critical view. Given his sharp intellect and his own deep understanding of the wizarding world and the characters in it. If he reads the epilogue fanfiction, he might scoff at any inaccuracies or misinterpretations of the characters' personalities. He could also be a bit sad, thinking about how things could have been different for him in the future depicted in the fanfiction.
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's reaction would be complex. He has so many emotions tied up with Snape already - dislike, confusion, and a bit of grudging respect in some ways. Discovering he's his father would bring all those emotions to the surface and more. He might go through a period of denial, not wanting to believe it at first. Then, as the truth sank in, he'd have to come to terms with a new family member. He might also feel a sense of betrayal from those who knew or suspected but didn't tell him. And he'd have to figure out how to fit this new relationship into his life, especially considering his role in the wizarding world and his ongoing battles.
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.