Some of the younger students who looked up to Harry, like Neville, could support him. Neville has grown to be a strong and brave character. He might see Harry's actions as a way to make the wizarding world a better place. Also, those students who have felt wronged by Dumbledore's decisions in the past, perhaps in terms of school policies or safety, could come forward to support Harry in the court case.
Hermione would be a likely supporter. She is always loyal to Harry and if he presented her with evidence of Dumbledore's wrongdoings, she would probably side with him. She has the intelligence to help Harry strategize and find ways to expose Dumbledore. For example, she could use her research skills to dig deeper into Dumbledore's past.
One possible plot could be that Harry discovers some long - hidden secrets about Dumbledore's actions during the war. Maybe he finds out that Dumbledore knew more about Voldemort's plans but didn't act in a way Harry thought was right. So, Harry decides to take him to court to expose the truth and seek justice for those who suffered because of what he believes were Dumbledore's missteps.
The Order of the Phoenix members might also back Harry. They've fought against the dark forces before and if they see that the Ministry has become a threat, they'd side with Harry. Dumbledore's old allies in the Order would believe in Harry's judgment and join him in his mission to destroy the corrupt Ministry.
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 is often one of the main supporters. Her intelligence and knowledge of magic are always at Harry's disposal. She can help him research spells and strategies when he takes charge of a situation.
Dumbledore himself could be seen as the main antagonist in this scenario. Since Harry is set on taking him down, Dumbledore's actions, whether they are truly malicious or just misinterpreted, would be the main obstacle for Harry to overcome.
I'm not sure specifically as I haven't read that exact fanfiction. But it could potentially be about Harry Potter having some sort of conflict with Dumbledore and taking actions against him. Maybe Harry discovers some dark secrets about Dumbledore that make him question his leadership and take steps to oppose him in the fan - created storyline.
Perhaps it's a girl orphan who was found wandering near the Forbidden Forest. She has no memory of her past but shows great potential in potions. Harry feels a connection to her because she seems so lost, just like he once was. So, he decides to take her in as the first orphan in his new orphanage.
Maybe in the fanfiction, there is a misinformation or a spell that has corrupted Dumbledore's mind. Harry, being the hero he is, notices the change in Dumbledore's behavior. He decides to take down Dumbledore because he thinks it's the only way to save the wizarding world from the new, dangerous path Dumbledore seems to be leading them on. For example, Dumbledore might be making alliances with dark forces unknowingly due to the spell, and Harry has to stop him.
Well, there are several reasons. Dumbledore is this all - knowing figure in the original series, but in fanfiction, his actions can be seen as manipulative. He might be making decisions for Harry without really considering Harry's feelings or wishes. Also, Dumbledore's grand schemes might not always be in Harry's best interest. Harry might realize this and get fed up. For instance, if Dumbledore is using Harry as a pawn in a bigger battle against Voldemort, Harry would likely be angry when he figures it out. And Dumbledore's failure to prevent certain tragedies, like the death of Sirius, could also be a reason for Harry's disillusionment.