Well, he'd probably be really confused at first. I mean, Hogwarts is all about magic and Forks High School is a normal Muggle school. He might accidentally use magic when he's frustrated with a difficult class like Potions... I mean, Chemistry. And he'd surely stand out with his lightning - shaped scar.
Well, perhaps Harry could be a wizard - creature hybrid. In Forks, he might discover that the vampires there have a connection to the magical world he comes from. Maybe he uses his magic to help the Cullens with some sort of threat that normal vampire powers can't handle.
In the fanfiction, if Harry Vampire goes to Forks, he could potentially fall in love with a human there, just like Edward did in the original Twilight story. But his vampire nature might make things more complicated. He could have to struggle between his love for the human and his vampire instincts. Also, he might face challenges from the local vampire community if they don't accept an outsider like him.
He'll most likely meet Bella Swan. Maybe he stumbles upon her in the forest and she's initially very wary of him, but then they start to talk and find they have some similarities in terms of being outsiders in their own worlds. He could also meet the Cullens. Edward might be intrigued by Harry's magic and Harry could be fascinated by Edward's vampire abilities.
Well, perhaps Harry would struggle to fit in at first. He might be shocked by the normalcy of Forks High School compared to Hogwarts. Maybe he'd make new friends and introduce them to some of his magical knowledge in a very discreet way.
Perhaps Harry goes to Forks because he senses some dark magic there that is related to the Volturi. He could end up getting involved in their power struggles. He might also start to learn about the unique rules and powers in the Forks supernatural world and compare them to what he knows from the wizarding world. For example, the way vampires have their own kind of 'magic' with their super strength and speed.
Definitely the Cullens. Since they are the main vampire family in Forks, they could be great allies. They have their own set of powers and knowledge of the area. Harry could also befriend Bella, who has her own connection to the supernatural world in Forks.
Obviously, Harry is one of the main characters. And depending on how the fanfic is written, there could be some characters from the Forks world that are also main characters, like Bella or Edward if it's a crossover type of fanfic. There might also be some original characters created specifically for this fanfic to interact with Harry.
It would probably be a wild mix. The female Harry Potter would be this outsider with amazing powers in Forks. She might befriend some of the locals, like Jessica or Angela. And when it comes to the vampires and werewolves, she could either be a threat or an ally. For example, she could use her magic to create some sort of protection spell around Forks to keep the vampires and werewolves in check.
There are many potential characters. The teachers at Forks High School for sure. Harry could have interesting conversations with them. Also, if there are any other supernatural beings in the Forks universe in this fanfiction, like werewolves, he could interact with them. And of course, the students who are into different clubs and activities at the school. He might bond with the science - loving students or those on the school newspaper team, sharing his own experiences from Hogwarts in a veiled way.