Obviously Naruto himself is the central character since the story is about him losing his memories. Sakura would likely be involved too. She's one of Naruto's closest friends and a medic - ninja, so she might try to find a way to restore his memories.
Dumbledore could also play a role. He might see Harry's weight loss journey as part of his overall growth and offer some cryptic yet inspiring words. Maybe he arranges for Harry to have special training sessions with the ghosts of Hogwarts, who have unique ways of making exercise fun. And of course, the Hogwarts professors like Madam Pomfrey could be involved, ensuring Harry's health during his weight loss process.
Sakura might also be a main character. In the original series, she is part of Naruto and Sasuke's group. Her skills as a medical ninja could be crucial in a story that involves future memories. Maybe Naruto's future memories show a situation where Sakura's medical skills are the key to saving someone important. Additionally, Kakashi could be in it too. His experience and wisdom could help Naruto make sense of his future memories and figure out what actions to take.
Itachi might be involved. Even though he has his own complex backstory, in this fanfiction, his powerful abilities and knowledge could be used to try and stop Naruto from losing his humanity completely. And the yaoi aspect could come in as Itachi forms a deep, perhaps unorthodox bond with Naruto during this process. Another character could be Neji. He has always been somewhat of a rival and friend to Naruto. Here, he could be the one who is determined to bring Naruto back to his normal self, and a yaoi - themed relationship could potentially develop between them as Neji delves deeper into Naruto's situation.
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.
Naruto is the central figure in this 'naruto imprisoned by his family fanfiction'. His family, including Minato and Kushina, are likely to be directly involved in his imprisonment. However, other characters from the Naruto universe might also play a role. For instance, Jiraiya could be a character who tries to find out why Naruto has been imprisoned and works towards his release. Also, Sasuke might get wind of Naruto's situation and decide to take action, either to help or because he sees it as an opportunity to gain an advantage.
Obviously Naruto himself is the central character as it's about his hibernation. His friends like Sasuke and Sakura are also likely to be involved. They would probably be the ones most concerned about his condition and try to figure out what happened.
Clary is the central figure. Valentine might also be involved in some fanfictions, either as an obstacle trying to prevent Clary from getting her memories back because he doesn't want her to discover certain secrets, or in a more complex way where his past actions are somehow related to the loss of her memories in the first place. And of course, the Lightwood family members are likely to be involved, given their importance in Clary's life and the Shadowhunter community.
Luna Lovegood could be a character in such fanfiction. Given her unique perspective on things, she might have an interesting take on Snape's memory loss. She could offer some unorthodox solutions or just be a calming presence in the chaos that follows Snape's changed state. And of course, Dumbledore might be involved as he has always had a complex relationship with Snape and would surely be interested in how this memory loss affects Snape and the future of Hogwarts.
Naruto is the primary one for sure. But there could be other characters like Jiraiya. Jiraiya is known for his knowledge of various things in the Naruto universe, and he could play a role in teaching Naruto about demon breeding. Additionally, some of the demon characters themselves could be considered main characters in a sense. They would have their own personalities and goals, and their interactions with Naruto would be a major part of the story. For example, a powerful demon that Naruto is trying to breed might have a complex backstory and motives that drive the plot.
In this fanfiction, Naruto is the key figure. However, characters like Shikamaru, who is known for his intelligence, might also be involved. He could be helping to plan Naruto's welcome or dealing with any potential issues that Naruto's return might bring. Also, Hinata, given her relationship with Naruto, would surely be part of the story. And the elders of the village may also play a role, as they often have a say in important matters regarding the village and its shinobi.