Hermione and Ron would most likely be his main supporters. They've always been by his side through thick and thin.
The students of Hogwarts might also back Harry up. They know Harry well and have seen him face many challenges before. They would understand that if Harry is suing the Ministry, it's for a good reason. Some of the more outspoken students like Luna Lovegood could use their unique perspectives to rally support for Harry among their peers.
I think the Order of the Phoenix members could also be his supporters. They believe in fighting against the wrongdoings of the Ministry and would support Harry in his lawsuit. Dumbledore, if he were still alive in the fanfic, would probably be a very important figure in guiding and supporting Harry as well. After all, he always had a strong influence on Harry and his decisions.
Maybe Harry Potter sues the Ministry for false accusations or for not taking proper actions against dark forces. It could also be about some new laws the Ministry made that directly harm Harry or his friends.
Hermione could be a key character. She is always by Harry's side, and her intelligence and knowledge of the wizarding laws would be invaluable in the lawsuit. She might be the one doing all the research and preparing the legal arguments for Harry.
Maybe Harry Potter decides to sue the Ministry because they have been withholding important information about his family or his true destiny. It could also be that the Ministry has been unjustly persecuting him or his friends, and he decides to take legal action within the wizarding world's framework.
Hermione would most likely be one of his top allies. She's smart and well - read in wizarding law, so she could help Harry build his case. Ron would also be there for moral support and to offer his own unique perspective on things.
In this fanfic, Harry could be suing the Ministry over their education policies. For example, Hogwarts might be facing some unfair regulations that are harming the students' learning experience. Harry, having a strong sense of justice, takes on the Ministry. He might gather support from other students, teachers, and even some of the wizarding families who are also affected by these policies. He would then present his case in a wizarding court, which could be full of all sorts of magical procedures and challenges.
In a fanfic, Harry Potter could inherit the Ministry. There could be some ancient wizarding law that gives him the right to own it. For example, if he is related to an old and powerful wizarding family that has a claim to the Ministry. He might not even be aware of this at first, but as the story unfolds, he discovers his right to own the Ministry.
His friends from Hogwarts like Hermione and Ron would most likely support him. They have always been by his side through thick and thin in the original story, so in the fanfiction, they would continue to support Harry in his fight against the Ministry.
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.
He might meet Hermione. Since Hermione is always interested in academic things like Owls, she could be there to help Harry or just be involved in some research related to it.
It could be that the fanfiction used Harry's likeness without permission in a way that was harmful to his image. Maybe it had Harry involved in illegal or immoral activities that he would never do in the original series, so he decides to take legal action against the author of the fanfic.