Detective Igarashi, whose full name is Daisuke Igarashi, is a supporting character in the Ju-on: The Grudge film series. He is a police detective whose investigation into a series of brutal murders leads him directly into the path of a supernatural curse. Igarashi consistently works alongside his colleague and partner, Detective Nakagawa, as the two of them form a pair of rational, professional investigators attempting to solve cases that defy logical explanation.

In his professional capacity, Igarashi is first seen responding to the scene of a horrific crime at a house in Tokyo, a location later identified as the site of the Saeki family murders. He and Nakagawa conduct a thorough investigation of the residence, leading them to a grisly discovery in the attic where they find the bodies of multiple victims. His investigative approach is methodical; as the case unfolds, he actively gathers background information. For instance, Igarashi is the one who compiles and shares the police reports concerning the original Saeki family, understanding that the history of the house is the key to understanding the present-day deaths.

His primary motivation is rooted in standard police procedure and a sense of duty. He seeks to solve the mystery of the Tokunaga family killings (or the Williams family in the American remake) by tracing the house's dark history. This leads him to recruit a valuable resource: Yuji Toyama, a former detective who was the lead investigator on the original, unsolved Saeki case. Igarashi recognizes that Toyama possesses unique and necessary knowledge, demonstrating a pragmatic drive to solve the case using any available means. His role in the story is that of a supporting figure who expands the investigation's scope, moving from the immediate crime scene to the historical record of the curse. His key relationship is with his partner, Nakagawa, with whom he works in tandem, sharing information and leads, such as when Igarashi informs Nakagawa about the disappearance of a key witness and the related death of a security guard.

Regarding character development, Igarashi, like most characters in the franchise, is defined more by his function in the narrative than by a personal arc. He begins as a competent, unflappable detective confident in his methods. However, his journey is one of being gradually drawn into a supernatural phenomenon he cannot process. The critical moment of development comes when he and Nakagawa respond to the Saeki house to find the disturbed Toyama. Instead of solving the case, Igarashi finds himself confronting the ghostly force directly. His fate is sealed in this confrontation, as he and Nakagawa attempt to rescue Toyama but are instead overtaken by the curse themselves, becoming its latest victims. No notable abilities or special skills beyond the standard deductive and investigative techniques of a police detective are mentioned in the source material.