Obviously, Harry Potter himself is a key player. He is the one on trial and has to defend himself. Hermione and Ron would also be important. They would be by Harry's side, helping him gather evidence and coming up with arguments to prove his innocence.
Well, in the 5th year trial fanfiction, Harry might face some unjust accusations. Maybe the Ministry of Magic is trying to pin something on him again, just like in the original story. He could be put on trial for using magic outside of school when he was actually defending himself.
In this fanfiction, a key witness might be Bellatrix Lestrange. If she were alive, she could be made to testify about the Malfoys' involvement with Voldemort. She was very close to the Dark Lord and knew a lot about the inner workings of the Death Eaters. Also, members of the Order of the Phoenix, like Remus Lupin or Kingsley Shacklebolt, could be witnesses. They were constantly fighting against the Death Eaters and would have knowledge of the Malfoys' actions. Additionally, some of the house - elves who served the Malfoys could be witnesses, as they would know a great deal about what went on in the Malfoy Manor.
Dumbledore, even if he's not physically present (maybe he's passed away in this scenario), his influence on Harry would still be a factor. Harry might be guided by the lessons Dumbledore taught him. And then there could be some new characters, like a young, idealistic lawyer - type wizard who believes in Harry's cause and decides to represent him at the trial.
Well, obviously Harry Potter would be a key witness. He was at the center of many of Umbridge's plots. Hermione would also be there, presenting evidence of academic suppression. But don't forget about Ginny Weasley. She was part of Dumbledore's Army and could testify about how Umbridge tried to break up the group. And then there are the Hogwarts professors. McGonagall, for instance, could give her account of how Umbridge disrupted the normal running of the school and undermined the authority of the real Hogwarts staff.
Obviously, Harry Potter is a key character. He is the one on trial, so the whole story revolves around him trying to prove his innocence.
Luna Lovegood might also be a possible love interest. Her unique personality and view of the wizarding world could attract Harry in a different way. She's not afraid to be herself and is very open - minded. In an AU, Harry could see her in a new light and they could start a relationship based on their shared experiences of being a bit different from the norm in Hogwarts.
Well, if the trial is about a Hogwarts student who is in trouble, then Neville could be one of the main characters. Or if it's a trial related to the dark side, Draco Malfoy could play a significant role. It's also possible that some minor characters from the original books, like Luna Lovegood, could become main characters in this fanfiction if they are somehow closely related to the trial, for example, if they have crucial evidence or are the ones being accused.
A magical creature that has the ability to speak and befriend Harry would be a great addition. For instance, a phoenix - like creature that is not as well - known as Fawkes. It could offer Harry advice and help him in the Triwizard Tournament, and their friendship would be a heartwarming aspect of the story.
In a Harry Potter Umbridge trial fanfiction, Umbridge could finally be held accountable for her cruel actions at Hogwarts. Maybe she'd be tried for her abuse of power, like using the blood - quill on students. The Wizengamot might hear testimonies from students like Harry, Hermione, and Neville about the unjust punishments and the way she ran the school tyrannically.
A 'Harry Potter Malfoys trial' fanfiction could have many twists. For example, the evidence against the Malfoys might be fabricated by some who want to take over their wealth and status. The Malfoys could then fight back, with Draco using his intelligence to prove their innocence. Hermione might be involved as a legal advisor of sorts, using her knowledge of wizarding law. And Harry could be conflicted, as he has his own history with the Malfoys but also believes in justice. The trial could also expose the dark underbelly of the wizarding legal system, with corruption and bias playing a role.