Neville might be an unexpected supporter. He has grown a lot in courage and magical ability. He could offer his unique perspective on plants and Herbology - related magic. In a situation where Dumbledore least expects it, Neville's knowledge of magical plants could be the key factor. For example, Neville could use a plant's special properties to disrupt Dumbledore's magic or create a distraction that Harry can take advantage of.
Maybe Harry discovers a new spell that Dumbledore is unaware of and uses it to outwit him. For example, a spell that can reverse time briefly in a small area, allowing Harry to redo a crucial moment in their 'game' to his advantage.
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.
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.
For Harry, his friends like Ron and Hermione are likely to be his allies. They've always been by his side through thick and thin in the original series, so it's natural for them to support Harry in a confrontation with Dumbledore. Neville could also be an ally, given his growth and courage shown in the books.
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.
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.
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.
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.
Hermione would be a likely candidate. She is Harry's best friend and is very intelligent. She could use her knowledge of spells and magic to come up with a plan to help Harry escape. For instance, she might create a diversion or find a way to disable the security around Dumbledore's area.
The main characters would be Harry and Dumbledore for sure. But it's possible that some of the other Hogwarts teachers, like McGonagall, could also be involved. She might be a witness to the confrontation, or she could be someone Harry goes to for advice before or after he tells Dumbledore off. Additionally, Snape might also play a role, as his relationship with both Harry and Dumbledore was quite complex.