Kuraudo Ōishi is a veteran detective with the Okinomiya Police Station, born and raised in the city of Okinomiya. His early life was shaped by loss during World War II, when the Nagoya factory where his real father worked was bombed and his father was killed. Ōishi did not get along with his father as a child but still felt the absence deeply. In the harsh years after the war he was assigned to crack down on illegal food purchases on the black market, work he found shameful. During that time he met a man who became like a second father to him. This man later served as the manager of the dam construction project, and in a fit of rage during a fight with drunk workers he turned on them violently and was subsequently lynched and dismembered. Because his death was later counted as the first in the string of annual mysterious deaths tied to Oyashiro-sama’s curse, Ōishi came to believe that the Sonozaki family, who had led the anti-dam movement, was responsible. That belief hardened into a decades‑long fixation: for the past five years he has relentlessly tried to prove that the Sonozaki family is behind the strange deaths that occur around the time of the Watanagashi Festival, growing more persistent as his retirement draws near.

Ōishi is a physically sturdy, somewhat heavyset man in his mid‑fifties with short grey hair and green eyes. He is almost never seen without a cigarette, smoking chain‑style even where it is prohibited, and often wears a cream‑colored suit with suspenders, a black collared shirt, and a red tie, frequently carrying his jacket over his arm in the heat. On the surface he presents a light‑hearted, genial, and at times playfully shady demeanor. He speaks with a syrupy, evasive delivery and punctuates conversations with a knowing, ambiguous laugh. This affable front makes him seem approachable and allows him to gather information without immediately raising alarm. Beneath that façade is a deeply resolute and calculating investigator. His dedication to solving the serial murders can tip into intimidation and violence when he feels pressed, as shown when he does not hesitate to get physical during interrogations. At the same time, his loyalty to his case can make him self‑interested, such as when he tried to exploit Keiichi Maebara’s fragile state to further his own investigation.

His drive to uncover the truth is intensely personal. The victim of the first murder in Hinamizawa was a close friend with whom Ōishi regularly played mahjongg, and that loss is what first pulled him into the annual mystery. He is an avid mahjongg player and occasionally enjoys games with Keiichi. Beyond that personal vendetta, Ōishi feels a profound need to bring the case to light, not only for his own closure but also to prevent the events from fading into obscurity. Even after reaching retirement age, he remains determined. He moves his elderly mother to Hokkaido, fulfilling a long‑held wish of hers, yet he does not abandon the investigation. Together with his friend and fellow investigator Mamoru Akasaka, he co‑authors a book titled “Higurashi no Naku Koro ni” that compiles all the documented events in Hinamizawa, hoping that one day the truth will finally be understood.

Within the story he serves as the outsider‑insider link, the one adult who openly seeks answers to the village’s secrets while operating from the relative distance of the police force. He frequently approaches the main cast, notably Keiichi Maebara and later others, to act as his informant. While his intentions are to solve the case, his probing questions sometimes accidentally feed the paranoia of those already on edge, unwittingly pushing them toward catastrophe. The villagers largely despise him: because he suspects the entire community and particularly the Sonozaki family, he is seen as a nuisance, especially by Mion Sonozaki, who fiercely believes in her family’s innocence. In a dark irony, many villagers suspect that Ōishi himself might be the culprit, calling him “Oyashiro‑sama’s servant,” precisely because he was observed making contact with each victim shortly before their deaths.

Over the arcs, Ōishi’s character reveals considerable depth. He is a capable hand‑to‑hand combatant, skilled in wrestling and aikido‑like techniques despite having never actually fired a gun. His investigative methods rely on observation, deduction, and a patient, almost stubborn refusal to jump to conclusions without concrete evidence. After retirement he takes up ballroom dancing as a way to stay physically fit, a detail that shows a softer, more personal side that contrasts with his gruff professional persona. His friendly rapport with Mamoru Akasaka grows into a true partnership, and together they become a moral counterweight to the cycle of tragedy, trying to preserve memory and find justice even as the village continues to be consumed by fear.