Yotsuya occupies room 4 of a boarding house, his enigmatic presence defined by eccentric habits and an intentionally obscured past. No concrete details about his age, occupation, or personal history emerge, though he asserts employment and adheres to irregular hours. A familial tie to the boarding house surfaces through an attic-discovered album, anchoring his lineage to the property.

Middle-aged with a perpetually deadpan demeanor, he dresses in formal dark grey suits or grey yukatas, contrasting his clandestine activities. He indulges in voyeurism, eavesdropping, and carving holes into neighboring walls—particularly room 5’s—to surveil or infiltrate. These breaches aid his theft of food, violations of privacy, and strategic leaks of others’ secrets to manipulate situations, such as obstructing repairs.

Interactions with fellow residents blend dry humor and orchestrated mischief. He allies with two female tenants, forming a trio that hosts drinking sessions and collectively harasses the protagonist. Though he favors one woman, often peering into her room unchallenged, his actions lack romantic undertones. During a pre-wedding event in the final chapter, he provokes by inviting the boarding house manager while retaining his detached air.

His name, a numerical pun translating to "four valleys," mirrors his room number, aligning with a broader resident-naming motif. Polished speech and attire clash with his chaotic antics, emphasizing deliberate incongruity. Unverified fan theories posit a cryptic link to a secret agent from another work by the same creator.

Across official narratives, his role persists unchanged: a static agent of disruption and comedy. His life beyond the boarding house remains unexplored, quirks unresolved, and veil of mystery intact.