His friends, like Ron and Hermione, are very likely to support him. They've always been there for him in the original story, and in this fanfiction scenario, they would be even more important in his life. Hagrid could also be a great source of support. He has a big heart and has always cared about Harry.
Ron and Hermione are almost certain to be by Harry's side. They understand him and his situation, and they'll offer him friendship and practical help. Neville could also step up. He's a kind - hearted character and might empathize with Harry's feeling of being neglected. Dumbledore, too. He's a very wise figure and might see it as his duty to guide Harry through this difficult situation of having neglectful parents.
Hermione would be a very likely character to support Harry. She's his best friend and is always there for him in the original story. Ron too, as they are a trio. Neville could also step up, as he has his own growth arc and could understand Harry's feeling of being alone to some extent.
Hermione Granger would surely be one of the first to support him. She's always been by his side, helping him with research and providing intelligent advice. Ron Weasley would also be there, being Harry's loyal friend through thick and thin. Neville Longbottom could be another supporter. He's grown into a brave and capable wizard, and he would stand with Harry in his new role. Dumbledore, if he were still alive in the fanfiction, would likely offer his wisdom and guidance to Harry as he takes on this new and important role.
Ron would also be there. He might start out a bit reluctant, but then get into it. He could be Harry's partner in physical challenges, like having races on broomsticks or wrestling magical creatures in a controlled environment for fitness.
Obviously, Harry Potter himself is the central character as the one being abused and neglected. The Dursleys are also key characters as they are the ones who abuse and neglect him in the original story. Hermione and Ron might also be involved as they could be the ones to discover Harry's situation and try to help him. Dumbledore could also be a part of it, perhaps as someone who failed to notice or prevent Harry's abuse initially.
The Weasley family as a whole might also support Harry. They have always treated Harry like one of their own. Arthur Weasley, being an open - minded wizard, might understand Harry's perspective. Ginny, who is very close to Harry, would surely be on his side as well. They would see Harry as more than just a student defying a headmaster, but as someone fighting for what he believes is right.
Ron might support Harry at first. Since he's Harry's best friend, he would be on Harry's side without really knowing the full story. He'd probably offer to help Harry with some of his revenge plans, like distracting Ginny while Harry sets up a magical prank.
His friends from Hogwarts like Ron and Hermione. They are always by his side and would probably be more than happy to help Harry get revenge on the Dursleys. They might use their magic skills together to create some really interesting revenge scenarios.
Well, in such fanfictions, a frequent theme is Harry's emotional turmoil. He has to deal with the pain of being neglected by his parents, which can lead to him having trust issues. Also, his magical abilities might be explored in a different light. Maybe he has to learn to control his magic on his own without the parental guidance that he should have had. And his relationships with authority figures at Hogwarts could be affected too, as he may view them as potential replacements for his absent parents.
Another character could be Neville Longbottom. In some fanfictions, Neville is neglected by his family not just because they think he's not as magically talented as they hoped, but they actively try to suppress his magic. This can lead to Neville having a more difficult time at Hogwarts initially as he has to overcome both the lack of support from his family and his own self - doubt.
Ron and Hermione are the most likely. They've always been by Harry's side in the original series. In the fanfiction, when Harry stands up for himself, they would surely be there. Ron might use his sense of humor to diffuse the situation if it gets too tense, while Hermione would back Harry up with her vast knowledge of wizarding laws and rights.