Shuhei Matsumiya

Shuhei Matsumiya is a detective in the Tokyo Metropolitan Police Department whose careful reasoning helps connect the murder of Michiko Oshitani to a second case, the assumed identities of men living under false names, and Kyoichiro Kaga’s search for the truth about his mother. As Kaga’s cousin and a colleague who can test ideas with him, Matsumiya occupies a point where the investigation’s evidence-gathering and its private family history meet.

Identity and Background

Matsumiya is a member of the Tokyo investigative team and, at the outset of the inquiry, the youngest of the five investigators facing Fumihiko and Masako Oshitani in the conference room. The couple’s timid manner is set against the imposing appearance of the other men, while Matsumiya’s relative youth distinguishes him within the group. His position does not prevent him from bringing observations to senior officers or from being assigned work alongside experienced colleagues.

His family connection to Kaga is both personal and professional. Kaga is his cousin, formerly of the Tokyo Metropolitan Police Department Homicide Division and now assigned to the detective bureau of the Nihonbashi Precinct. The two had worked together on an investigation several years earlier. Matsumiya can therefore approach Kaga not only as a fellow detective whose judgment he respects, but as a relative whose unresolved family history is unfolding alongside the case.

Investigative Work and Reasoning

When Michiko Oshitani’s work pattern was reviewed, Matsumiya noticed that she had gone to work as usual through Friday, March 8, then stopped attending from Monday, March 11. He inferred that she had most likely been killed on March 9 or 10. He set that estimate beside the strangulation case in Shinkoiwa on March 12 and told the senior investigators that the two cases might be connected. The link was a hypothesis, not a conclusion, but it shows his characteristic use of chronology and method to give an investigation a direction.

The police were unable to establish who Mutsuo Koshikawa was from the contents of his apartment: it held no phone, photographs, identification, credit cards, or letters that could disclose his identity. Investigators suspected that the personal items had been deliberately removed. Koshikawa had lived there for nine years without updating his residence certificate, and his stated previous address in Maebashi, Gunma Prefecture, yielded no useful information and was probably false. The building’s lax management had allowed a tenant to remain without adequate vetting. The case thus required Matsumiya and his colleagues to treat official records as claims to be checked rather than reliable proof of identity.

The investigation gathered physical evidence as well as documentary leads. To test whether Koshikawa might himself have died, police took a toothbrush, safety razor, and old towel from his apartment for DNA comparison with unidentified deceased persons. The woman found there was also unidentified: a handbag and travel pack were near the body, but neither yielded the usual identifying documents. At a task-force meeting, the team decided to use fingerprints and DNA to identify her. These procedures form the evidentiary background to Matsumiya’s later work, in which identity is established not by a name alone but by comparing records, biological traces, and people’s recollections.

After the Oshitani family confirmed the victim’s identity, Matsumiya and Sakagami were ordered to travel to Shiga Prefecture. Their assignment was to investigate the firm where Michiko had worked and report on her friendships, relationships, and any links to Tokyo. Matsumiya also raised his concern that the timing and shared method of killing connected her death to the Shinkoiwa case. He was not simply following assigned inquiries: he was actively drawing together separate incidents and articulating why they might belong to one investigation.

A later forensic comparison gave the identity question a firm answer. Fingerprints recovered from an old train timetable matched prints from Mutsuo Koshikawa’s apartment, objectively establishing that Koshikawa and the man living in Sendai as Shunichi Watabe were the same person. The number and placement of the prints indicated that Watabe, rather than Kaga’s mother, had handled the timetable most often. Matsumiya accepted this distinction as important: it corrected an assumption that the object might primarily reflect the mother’s movements and showed how the distribution of traces could help distinguish between two people associated with it.

Matsumiya’s field inquiries also combine official leads with careful confirmation by witnesses. After new information identified former workers who had performed maintenance at Hamaoka Nuclear Power Station and Onagawa Nuclear Power Station alongside Kazutoshi Yokoyama, investigators located an address for Sadakichi Nozawa. Matsumiya travelled to see him, reaching a low-rise apartment block after taking a train to Shinmaruko Station and walking through its small shopping streets. Nozawa confirmed that he had worked with a Yokoyama at Hamaoka and recognized the man in a photograph as someone who always looked miserable and rarely smiled. The recognition did not prove the entire case, but Matsumiya understood its value because Nozawa’s response resembled that of Yasuyo Miyamoto, another witness. Nozawa further distinguished their work at Onagawa: he worked for a power-generation subcontractor, while Yokoyama worked for Watabe. The converging recollections strengthened Matsumiya’s confidence without turning corroboration into certainty prematurely.

Kaga, Family History, and Personal Stakes

Matsumiya’s relationship with Kaga makes him a witness to the personal burden carried by the detective leading parts of the investigation. When Kaga explained that his father, Takamasa Kaga, had believed Yuriko had done nothing wrong and had thought she might want to see her son before she died, Matsumiya recognized the account. He recalled that Kaga and his father had agreed Kaga would not visit even when Takamasa was critically ill, because the father had chosen to die alone; Kaga had not been in the room when he died. Matsumiya’s recollection connects the present inquiry into Yuriko’s life with an earlier, painful separation within the family.

The cousin’s perspective sharpens when the two visit the derelict place where Kaga’s mother had lived and died. Matsumiya finds it difficult to accept that his aunt had lived in surroundings that seemed to mean nothing to her. He remembers Takamasa’s belief that she had wanted to see her little boy before her death, and the physical pain he had described at the thought. The location does not resolve Kaga’s questions, but Matsumiya’s response gives the setting emotional consequence: it is not merely a point in the investigation, but a place that makes the cost of the family’s long estrangement difficult to ignore.

Kaga’s search is for more than the bare circumstances of his mother’s death. He tells Tokiko Kanamori that he wants to know what emotional state his mother had been in. Tokiko answers with confidence that Kaga was the person his mother had been thinking of when she died, because he was her only child. Matsumiya’s presence in the broader family story helps frame that search as an attempt to understand a relationship that endured despite absence, not as a simple effort to recover a missing fact.

Tracing the False Identities

Matsumiya’s work takes place amid a case in which legal and biological identity repeatedly diverge. An address Kaga had provided to a kendo magazine after winning the national police championships in judo and kendo might explain how his mother had later obtained his contact details. Kaga had withheld an inquiry to the publisher because he did not want to exploit his rank for a personal matter, and he feared that mentioning a murder might make the publisher less forthcoming. To ask about the magazine without revealing the case, Kaga and his colleagues presented their inquiry to Machiko Yoneoka as a survey of the Tokyo Metropolitan Police Department’s public-relations outreach and the impact of old newspaper and magazine articles. The magazine interview became one possible route through which a private address could have travelled, illustrating Matsumiya’s investigative world: information has a source, but the source must be approached without prematurely exposing what the police know.

The deeper identity puzzle concerned Shunichi Watabe and Hiromi Asai’s father, Tadao Asai. Kaga inferred that the recorded account of Tadao’s supposed suicide was suspect: the only record was in the files of Biwa Gakuen Orphanage, it lacked official documentation, and Hiromi’s former classmates did not remember the death. The Hokuriku Daily article reported that a body had been identified through fingerprints on luggage and visual identification by Tadao’s daughter, but Kaga reasoned that another man might have died under Tadao’s identity while Tadao survived. He asked Hiromi questions face-to-face, read her reactions as support for his hypothesis, and obtained a sample of her hair for DNA testing against Koshikawa, also known as Watabe. At this point the test result remained pending; the investigation had not yet converted the hypothesis into a confirmed biological relationship.

The earlier substitution had been deliberate. Tadao told Hiromi that he had arranged for another man to take his place, placing the man’s fingerprints on items in his bag and having him grip its handle so the dead man would be identified as Tadao. He asked Hiromi to tell the authorities that the body was her father. Tadao framed the deception as an escape from debt collectors and a way for officials to take responsibility for his daughter, even if that meant sending her to an orphanage. His plan explains why an apparent death could have erased one identity without ending the life attached to it; the later police inquiry must reconstruct that concealment from fingerprints, documents, DNA, and witness memory.

Role in the Narrative

Matsumiya’s defining function is to keep inference accountable to evidence. He proposes a connection between two killings by comparing dates and method; the team tests identity through fingerprints and DNA; and he treats a witness’s recognition as meaningful but not conclusive. The investigation advances through these incremental distinctions: a false residence address is not a reliable history, a name is not proof of a person, and a strong hunch still needs corroboration.

At the same time, his kinship with Kaga prevents the procedural plot from remaining emotionally detached. Matsumiya remembers what Kaga’s father said, recognizes the weight of Kaga’s absence at his father’s death, and sees the place where Yuriko lived through the perspective of a family member. The case’s chain of identities—Koshikawa and Watabe, Tadao and a dead stranger—runs parallel to the question that matters personally to Kaga: what his mother felt, and whether she continued to think of him. Matsumiya does not answer that question for his cousin. His narrative importance lies in helping the investigation establish what can be proven while standing close enough to understand why the remaining uncertainties matter.