Dumbledore could make an appearance in the form of a guiding spirit or through memories. His wisdom could be crucial for Harry as he navigates the new situations outside of town.
Hermione would be a common character. She would likely be very determined to find Harry and bring him back. She might use her vast knowledge of magic and research skills to track him down.
Hermione might be a common character. Since she's Harry's friend and very intelligent, she could be summoned along with Harry or find a way to follow him. Dumbledore could also make an appearance in some stories. He's very powerful and wise, so he might show up to guide Harry in the new world. Ron could be another one. His loyalty to Harry might lead him to try to rescue Harry from the other world.
Ron could also be there. He might be a bit jealous at first if Harry's adopted family is wealthier or has more influence in the wizarding world. But later, he would support Harry as always.
Hermione would definitely be one of the main characters. She could be the one to research all about braces in the wizarding library to see if there are any magical implications or ways to make the process easier for Harry. And she would offer him lots of advice and support.
Ron would also be a main character. He might start off by making fun of Harry's braces, but then he would stand up for Harry when some of the Slytherins start to bully him because of it. Ron could also try to use the braces to his own advantage in some comical ways, like using them as a slingshot for Bertie Bott's Every Flavour Beans.
Hermione could still be a common character. Even in fanfiction where Harry is alone, Hermione might be mentioned in his thoughts or in flashbacks as someone who has influenced him. For example, he might recall her advice on how to study magic when he is facing a difficult spell alone.
Draco Malfoy is a very common character. Since they are in the same house in this fanfiction concept, their interactions would be different from the original story. There would likely be more opportunities for them to either become friends or rivals in a new way.
There could be some original characters created by the fanfiction authors. These could be Harry's new classmates or friends at Durmstrang. They might have their own unique magical skills and personalities, and through their interactions with Harry, they can help develop the story further. For example, a new friend could introduce Harry to a secret part of the Durmstrang grounds or a special magical technique that's unique to the school.
Often, there are advisers or mentors. These are characters who have a lot of knowledge about the monarchy and its workings. They might be old nobles or wise scholars who help Harry understand the ropes of being the heir. There could also be antagonists like power - hungry nobles who see Harry as a threat to their own ambitions and try to get rid of him.
Hermione is often a common character. She would probably be by Harry's side, using her vast knowledge to help him understand his role as the Gryffindor heir. Ron might also be there, providing his loyalty and comic relief in tense situations.