In a lot of these fanfictions, Luna Lovegood can be a key character. She has an open - minded and unique way of looking at things. So she might be the one who first suspects that Harry isn't really dead, or she could be part of the plan to help Harry fake his death and then communicate with him in some secret way while everyone else thinks he's gone.
In many fanfictions, Kakashi could be involved. As Naruto's former teacher, he has a deep connection with him. He might be the one who discovers some strange things about Naruto's 'death' and starts to look into it further. Hinata is another character that could be important. Her love for Naruto might drive her to not accept his death easily and she could play a part in uncovering the truth.
Harry is the main one as it's his faked death. Hermione, with her sharp mind, would be a mainstay in the story. Ron, with his loyalty and bravery, would be right there. Neville could also be a main character. He has grown a lot throughout the series and could be involved in the events surrounding Harry's faked death, perhaps even being one of the first to suspect something fishy. And then there's Ginny. As Harry's love interest, she would be deeply affected by his faked death and would be a central figure in the emotional turmoil of the story.
For Ron and Hermione, they would likely be devastated at first, thinking they had lost their best friend. But then they might start to suspect something when they notice small details that don't add up. Ginny would be heartbroken and might become more determined to fight against Voldemort in revenge. The Order of the Phoenix would be in chaos, with some members losing hope while others might start to investigate the circumstances of his 'death' more closely.
Princess Peach could also be a main character. In some fanfics, she could be the catalyst for Bowser's death. Maybe her capture by Bowser sets off a chain of events that ultimately leads to his end, or she could be the one who finally defeats him in a twist of the story.
The Death Eaters might think they've won at first. There could be chaos and fear among the good wizards. But it could also lead to a secret resistance growing stronger.
Obviously, Harry Potter himself is the central character as it's about him in diapers. Dumbledore often features as he plays a crucial role in Harry's life. Hagrid might also be there as he has a special connection with Harry. And of course, the Dursleys could be part of the story, perhaps in their begrudging care of baby Harry.
Well, obviously Harry Potter himself is the central character. Hermione and Ron are almost always involved too. They would be the ones trying to figure out what's wrong with Harry and how to fix him. Dumbledore might also play a major role, as he often has deep knowledge about magic and could potentially help Harry regain his sanity.
Obviously, Harry Potter himself is the central character. But often, his friends like Hermione and Ron may also be involved as they are closely associated with him in the original series.
Another possibility is that the main antagonist is a group. It could be a secret organization that Naruto discovers is plotting something big. Since they are so powerful and well - connected, Naruto fakes his death to infiltrate them and expose their plans from the inside.
Obviously, Harry Potter himself is always a main character. He is the one most directly associated with the concept of death in the original series, what with his parents' death and his multiple near - death experiences.