TW: mentioned of sh and abuse
Iruka, but he has CIP (Congenital insensitivity to pain) and that's why he doesn't work in the field often, it's a threat to him. He has to hide his illness and desperately wants to be like everyone else, to feel pain, which is why his abusive relationship with Mizuki is based on violence, which Iruka has become addicted to, because he hopes that one day he will finally feel something. Every time he just pretends it hurts, because Mizuki wants to see him scared and Iruka wants his own reactions to become real.
He takes fuma shuriken to protect Naruto, but the pain doesn't come and even after Mizuki's betrayal Iruka's need of trying to feel something doesn't die down, so he does it himself. He looks with dull eyes as the blood run down his body, not feeling any pain, his tears start to fall down as well. Not only that, but he misses Mizuki too, because with him there was hope and now he is all alone.
