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.
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.
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.
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.
Daenerys Targaryen could potentially be an ally. She is open - minded about different powers and has her own dragons. Harry's magic could be seen as another powerful asset. And Jon Snow, being a noble and brave character, would likely welcome Harry's help in the fight against the White Walkers and might offer his own military expertise in return.
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.
Maybe some unexpected allies could emerge. For example, a centaur who has witnessed the unjust actions of the court might come forward to help Harry. Or a goblin who has knowledge of the court's financial misdeeds could offer assistance. Harry's reputation for being a hero could attract all sorts of beings who want to see justice served in the court.
I think Hagrid would be a great ally. He loves all magical creatures and would be very accepting of Harry's new form as a winter spirit. He could introduce Harry to some winter - adapted magical beasts that could help him. Also, some of the older students like the Weasley twins could be allies. They would be excited about Harry's powers and use their creativity to come up with new ways to use his winter magic for fun and for the good of the school.
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.