In addition, new characters might be introduced in the fanfiction. These could be healers or other wizards who are experts in dealing with mental issues. They could be brought in to try and save Harry from his downward spiral. Or perhaps there are some characters from the background of the original story who suddenly come to the forefront due to Harry's situation, like some of the other Gryffindors or teachers at Hogwarts who have to deal with the chaos that Harry's loss of sanity causes.
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.
Often in such fanfictions, the plot may involve a betrayal that pushes Harry over the edge. It could be a friend or a mentor who turns on him. This sudden shift in loyalty would shatter his world view. He has always relied on the support of those around him, and when that is taken away, it becomes difficult for him to stay sane. His actions would then become erratic as he struggles to come to terms with this new and cruel reality.
It could be Kaguya. If Kaguya were to return and use her powerful abilities to drive Naruto mad, perhaps by invading his mind with dark thoughts and memories. She has the power to warp reality and minds, so she could easily make Naruto lose his sanity. And since she has a grudge against the world of ninjas, making Naruto, one of the most powerful ninjas, go insane would be a great way for her to start her reign of terror all over again.
Perhaps an unknown and powerful dark ninja. This ninja could be from a long - forgotten clan with dark powers. They might have a vendetta against the Leaf Village and see Naruto as the key to their revenge. So they use their dark arts to make Naruto lose his sanity and turn him against his own village.
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.
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.
Maybe Orochimaru manages to get his hands on Naruto and subjects him to some dark and twisted experiments. These experiments could mess with Naruto's mind, causing him to lose his sanity. He could start seeing illusions all the time, believing his friends are his enemies. As a result, he'd be in a constant state of confusion and aggression, and his once - positive attitude towards life and ninja - hood would be completely gone.
Other characters in the Naruto universe might also be affected. For example, his friends like Sakura and Kakashi would notice the change in Naruto. They might try to help him, but their attempts could be in vain because of the complex situation with the Fem Kyuubi involved. This would create new tensions and storylines as they struggle to deal with a Naruto who has lost his sanity and the mystery of the Fem Kyuubi's role in it.
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.
It could be Voldemort. Voldemort might use Harry's love as a weapon against him. He could either kill her or turn her against Harry. Another possibility is Draco Malfoy. Maybe Draco tricks Harry's love into thinking he's a better choice. Or it could be a new, unknown dark wizard who targets Harry through his love.