Kōichi is a high school student living with his family, including his younger brother Sōichi and his sister Sayuri. His primary motivation throughout the story is to find a quiet environment where he can concentrate on his studies, as Sōichi constantly torments him with loud noises, curses, and disturbing antics from the attic. This persistent disruption drives Kōichi to take action. He appeals to his father, who arranges for a contractor named Tagaisu to soundproof his room. The resulting construction is bizarre: a series of four nested rooms, each entered through a door, creating a cramped inner chamber that finally offers silence. Initially satisfied, Kōichi soon discovers that Sōichi has created hidden passageways in the gaps between the walls and continues to harass him from the ceiling. Kōichi’s personality is that of a frustrated but determined older brother; he is serious about his responsibilities and tries to maintain order, but his efforts are repeatedly undermined by Sōichi’s mischievous and supernatural inclinations. He does not undergo significant character development, instead serving as a straight man whose ordinary desires clash with the surreal horror of his household. His key relationship is with Sōichi, marked by a contentious and adversarial dynamic — Sōichi views him as a target for petty revenge, while Kōichi sees his brother as an obstacle. Apart from his normal human capabilities, Kōichi possesses no special abilities; his role is that of a victim caught in a cycle of domestic torment that escalates into absurdity.