There could be a mentor - like figure for Harry who tries to help him through this betrayal. This character might be an old wizard or witch who sees the truth and tries to make Harry understand that his family's actions are wrong. This character adds another dimension to the story as they are in contrast to Harry's betraying family.
It could also involve Naruto's future family in some time - travel or alternate reality scenarios. For example, if Naruto from the future travels back in time and his future family members come with him. They might have been corrupted in the future and their actions in the past, like betraying Naruto, could be part of a chain of events that led to that corruption. This would create a complex web of relationships and a unique take on the 'betrayed by family' concept.
Obviously, Harry Potter himself is the central character. Others could include Ron and Hermione, who have been by his side through many life - threatening situations and might also be in debt to him in some way, for example, if Harry saved them from a particular dark spell during a battle. Also, Neville Longbottom could be involved as Harry saved him from Voldemort during the Battle of Hogwarts.
Even some of the professors could be involved. Professor Dumbledore, with his vast knowledge and wisdom, might have some connection to the magical trunk. He could be the one guiding the students in their exploration of it, or perhaps he has left some clues for them to discover. Professor McGonagall might also get involved if she senses that the students are in over their heads with the trunk's secrets and she needs to step in to protect them.
Hermione and Ron would most likely be main characters. They are Harry's closest friends in the original story. Since Harry is deaged, they would play important roles in taking care of him or guiding him in some way.
Obviously, Harry Potter himself is the central character. Since the story is about him losing his sanity, he is at the core of the plot. Then there would likely be his friends like Hermione and Ron. Their reactions to Harry's deteriorating mental state would be important aspects of the story. They might try to help him, or they could be at a loss about what to do.
Hermione could be involved. She might suspect Daphne of having ulterior motives regarding Harry and try to warn him, which could play into the betrayal storyline in some way, either as a catalyst or as someone who helps Harry deal with the betrayal later.
Obviously, Harry Potter himself is often a central figure. His friends like Ron Weasley and Hermione Granger also play important roles as they are part of his extended family - like group. Ginny Weasley, as Harry's wife in the original series, is also a key character. Then there are the older generation characters such as Molly and Arthur Weasley who bring their wisdom and family values to the alliance.
Obviously, Harry Potter himself is a main character. He is usually the one who discovers or is in search of the family grimoire. Hermione Granger is also often involved as she helps Harry decipher the complex spells and ancient writings in the grimoire. Ron Weasley may be there too, providing support and his unique brand of humor.
Obviously, Harry Potter himself is often at the center of it. His friends like Ron and Hermione might also be involved as they support him through the family drama.
Luna Lovegood could be one of the main characters. Her unique perspective on family and friendship could add an interesting dimension to the fanfiction. Neville Longbottom might also be involved. He has his own family story, and his growth and relationship with others could be explored in the context of family bonds. Additionally, Sirius Black could be a major character as he was like a father figure to Harry in some ways.