"The separation of the two categories (Neurology and Psychiatry) is arbitrary, often influenced by beliefs rather than proven scientific observations. And the fact that the brain and mind are one makes the separation artificial anyway."
Who is Fuminori Sakisaka? Fuminori is a med student studying Neurology. The ultimate irony is that he himself is inflicted with a horrific and nearly crippling neurological disorder which affects all five senses. He has Agnosia, which in less extreme cases would prevent someone from seeing a spoon as a spoon, being unable to identify words and give them meaning, being unable to perceive more than one object at once, etc.
Unfortunately, surgery following Fuminori's car accident left him unable to see, hear, smell, taste or feel anything normally. Everything Fuiminori sees is made of bloody flesh. If something is solid, it will have the texture and appearance of bone.(with the flesh splattered across it). He cannot read books, perceive light, or understand human speech entirely (it has an overlaying shrillness and buzzing to it but with effort he can understand them).
If someone spits at Fuminori, he perceives this as being vomited at, although from the other person's perspective it barely did anything. He sees human beings as rotting piles of tentacles with gaping mouths and lopsided eyes. He finds their company sickening, unbearable. He can't eat anything besides the most disgusting-- like raw human organs-- which look like golden succulent fruits.
Due to his disorder, in the three months he's had it he has lost all sense of empathy. He has come to accept that in his reality, he is the only human being left. The world outside his home is nearly impossible to traverse; in the novel he frequently mentions that if trains are crowded, he simply cannot ride them because of the stench from all these "fleshbeasts". He no longer has compassion for the thoughts, ideas or feelings of others. He does mention to Kouji in a brief glimpse of insightful conversation that he doesn't necessarily feel good about how he's treating the others. It's just how things are going to be. And that's all he can say.
Though Fuminori is capable of false kindness, it is easy for others to tell he's putting up a front. His eyes give away how tired and unhappy he is around others. He has no problem with faking his way into getting favors from others. He uses people to get what he needs, or what Saya needs.
And that is the Sun around which Fuminori's world spins: Saya. Originally a Lovecraftian horror from another dimension, Saya appears to Fuminori's warped senses as a beautiful young loli girl with long green hair. In actuality, Saya looks exactly the same as the flesh beasts he sees humans as. It's all ironic in so many ways, but Fuminori knows what she really is and doesn't care. To him, Saya is the only thing he has left. She is the branch he holds onto desperately to avoid falling into the chasm of complete insanity. Saya sees the world normally, but is aware of his disorder-- she spends her alone time learning to cook food for him, research brain patterns and Neurology, and of course- murder people for her to eat herself. Once Fuminori catches her doing this, she realizes that it's the only thing he can eat, and they begin sharing. (incidentally the first body Fuminori and Saya eat is Oumi, his best friend Kouji's lover. Kouji finds the 'leftovers for later' thawing in the fridge.)
Is there any hope for him? Yes and no. If there is any hope for Fuminori to become a better person, it's going to take time. As much of a complex monster he is right now, it is almost entirely due to the influence of Saya. By getting exposed to other people, he will eventually recover his lost humanity, and may even come to care about someone besides Saya, if her influence is lessened. The less time they're together, the healthier his mind will eventually become-- but anyone who becomes close to Saya will feel his madness very quickly. He's possessive, and just a bit ax-crazy.