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.
Hermione Granger might also be a main character. Her intelligence would be very useful in a smart trial scenario. She could be using her vast knowledge of magic and books to help someone during the trial.
In a 'harry potter veritaserum trial fanfiction', we might see Harry as the main character, especially if he is the one on trial. Hermione would likely be there to use her intelligence to figure out any legal loopholes or problems with the use of Veritaserum. Ron would be by their side for support. Additionally, Dumbledore or other Hogwarts professors could also be main characters, perhaps acting as advisors or having some hidden knowledge about the trial and the potion.
Obviously, Harry Potter himself would be one of the main characters. Since it's about his connection to the Lord Black trial. Sirius Black's presence might also be felt strongly, even if not physically present, as his past and the Black family's reputation are central to the trial. Hermione and Ron would most likely be there too, as Harry's closest friends, providing support and using their intelligence and skills to help Harry through the trial.
Obviously, Harry Potter himself is one of the most common. Then there's Dumbledore who often plays a role, either as a supporter or as someone with a complex plan. Hermione is also common as she uses her intelligence to help Harry in the trial.
In some Harry Trial fanfiction, the Ministry of Magic officials are common characters. For instance, Fudge or Scrimgeour might be involved in the trial as the prosecutors or judges. Their actions and decisions can greatly influence the outcome of Harry's trial, and their motives might not always be as straightforward as they seem.
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.
It could be about a trial within the Harry Potter universe created by a fan. Maybe it's a trial of a character for a crime they're wrongly accused of, like Harry being put on trial for using magic outside of Hogwarts again. It might explore new aspects of the wizarding legal system and how characters deal with it.
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.
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.