Harry Potter's reaction would depend on his mood when he finds the manga fanfiction. If he's in a relaxed state, he might be more open - minded and excited to explore this new form of storytelling. He'd notice the detailed art and different character designs. He may be a bit thrown off by the different magic concepts or lack of wands in the manga. But overall, he could grow to like it, especially if there are elements of adventure and mystery, which he loves. He might even start sharing some of the interesting parts he finds with his friends at Hogwarts, like Hermione, who would also be interested in analyzing the different narrative styles.
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.
Snape would probably be very conflicted. On one hand, he has a long - standing grudge against James Potter, Harry's father. But on the other hand, he loved Harry's mother, Lily. So, he might be cold at first, but then his sense of protecting Lily's son could kick in. He would likely be very strict with Harry, as he is with all his students, but also secretly keep an eye on him to make sure he's safe.
Snape would be deeply shocked and disgusted. Given his own experiences and the values he holds, finding Harry in a rape - related romance fanfiction would be like a blow to his perception of the world around Harry. He may initially be in disbelief, but then his anger would rise. He might try to find out who created such a disturbing fanfiction and perhaps try to put an end to it, as it goes against everything he believes in and the world he is a part of in the original Harry Potter series.
He might be surprised. The Harry Potter world has its own set of rules and characters that are very different from what Merlin is used to in his Arthurian world.
He'd probably be annoyed. Snape is a serious wizard and he might think fanfiction is a silly Muggle - influenced concept.
He might be curious. Harry is always eager to learn new things. He'd probably read it with great interest to figure out what this new world of RWBY is all about.
He may be confused at first. Fanfics can be very different from his real experiences. He might wonder why people write such things about him and his world.
He could be shocked. Maybe there are some fanfictions that take a very dark turn or have unexpected pairings that he never thought of. He'd probably need some time to process such strange ideas. And he might also be curious to see how far the creativity of the fans can go. He might even get some new ideas about his own life from reading some of the more imaginative fanfictions.
He'd probably be devastated at first. Gryffindor has been his home at Hogwarts, and his friends are there. But he's also a resilient character. He might start to question the fairness of the banishment and try to prove his innocence. He could look for allies outside of Gryffindor, like Luna from Ravenclaw, who might believe in him.
He'd probably be really excited. After all, it's a place that's dedicated to his world. He might be in awe of how Muggles have managed to capture so much of the magic.