There is a chance that it could be an older student who resents Harry for being in the spotlight. Older students sometimes pick on younger ones in Hogwarts. Or it could be a student who has some personal vendetta against Harry's family. Maybe their family has a history with the Potters and they are taking out their anger or jealousy on Harry in the form of bullying.
It could be Draco Malfoy. In the original series, he often bullied Harry, so it's likely he would be the main bully in this fanfic as well.
He could confide in his friends like Ron and Hermione. They would surely support him and come up with a plan to stop the bullying.
Another way Harry could deal with it is by getting the support of the Hogwarts faculty. He could gather evidence of the bullying, like spells cast at him without provocation or mean - spirited remarks. Then, he could present this evidence to a trusted teacher, such as Professor McGonagall. She would likely take action against the bullies, perhaps giving them detention or restricting their privileges. This would not only stop the bullying but also show that there are consequences for such actions at Hogwarts.
In fanfiction, it could be some of the Slytherin students. They might be jealous of Harry's fame as 'The Boy Who Lived' or his friendship with popular Gryffindors. Malfoy and his cronies are often depicted as bullies in the original series, so they are likely candidates in the fanfiction as well.
Hermione might go with him. She's always been by Harry's side, and she would likely follow him out of loyalty and also because she might see the value in whatever mission or journey Harry is embarking on. She could provide the intellectual support and research skills that Harry needs.
In the fanfic, Harry Potter could meet the new generation of students. They would be in awe of him. He might also encounter some of the ghosts, like Nearly Headless Nick, who would surely have some interesting stories to share since Harry left. And there's a good chance he'll meet the caretaker, who would probably give him a hard time about coming back and causing a stir. Moreover, he could cross paths with some of the older students who idolize him and want to learn from his experiences.
Perhaps they are three young wizards from a far - off land. They could be from a place where magic is practiced differently, like a tribe deep in the Amazon where they use magic to communicate with animals. So when they come to Hogwarts, they have a lot to learn and also a lot to teach. They might have unique magical abilities that are rare in the Hogwarts world, such as the ability to control the weather without a wand.
There could also be new characters introduced in the fanfic. Maybe a young, talented wizard or witch who is just starting at Hogwarts and becomes involved in Harry's return journey. They could bring a fresh perspective and new abilities to the story. And of course, Dumbledore, even if he's not physically present all the time, his influence and legacy are likely to play a role, so he can be considered one of the important characters in a sense.
Obviously, Harry would be one of the main characters. He is the central figure in the Hogwarts world. Hermione and Ron could also play significant roles as they are Harry's closest friends and are often involved in his adventures.
It could be a new dark wizard we haven't seen in the original series. Maybe someone who has a vendetta against Hogwarts and manipulates Harry into doing the dirty work of destruction.
Maybe he meets an old Canadian witch who has a lot of wisdom about a certain type of magic that is rare in the UK. She could give Harry some valuable advice or a magical item that helps him later at Hogwarts.