Mamoru Akasaka serves as a detective in the Tokyo Metropolitan Police Department's Public Safety Division. His story begins during the 1978 Himatsubushi-hen case, investigating the kidnapping of government official Toshiki Inugai's grandson in Hinamizawa. While his pregnant wife Yukie is hospitalized in Tokyo, he partners with local officer Kuraudo Ooishi, posing as a tourist. A young Rika Furude warns him to return to Tokyo immediately, predicting Yukie's imminent death and a series of murders culminating in her own death in 1983. Akasaka brushes off her warnings. After rescuing Inugai and suffering a gunshot wound, he finds all village phone lines cut, preventing contact. Returning to Tokyo, he learns Yukie died in a stairfall accident shortly after he left Hinamizawa; their daughter Miyuki survived. He later realizes Rika tried to spare him from receiving the traumatic news alone and that her predictions proved true. This failure haunts him, fueling profound guilt. In subsequent arcs, Akasaka's choices alter timelines. During Minagoroshi-hen in 1983, he visits Hinamizawa again, remembers Rika's warning, and immediately returns to Tokyo upon her urging. He arrives in time to warn Yukie about a hazardous staircase; she avoids an accident caused by a janitor's spill, ensuring her and Miyuki's survival. Akasaka later thanks Rika in Hinamizawa, pledging to protect her, though he departs before the festival, unaware of her immediate danger. His intervention directly changes this timeline. Akasaka's most significant evolution occurs in Matsuribayashi-hen. In one failed timeline fragment years after the Great Hinamizawa Disaster, consumed by guilt over failing Yukie and Rika, he becomes a skilled karate master dedicated to opposing injustice. Rika, aware of this version, transfers his combat skills and resolve to the Akasaka existing in 1983. This merged Akasaka initiates a Tokyo corruption probe, uncovering illegal funding to the Irie Clinic, which triggers memories of Rika's warnings. He contacts Ooishi, returns to Hinamizawa, convinces local authorities of Rika's predictions, and allies with Jirou Tomitake and Kyousuke Irie to counter the conspiracy against her. He plays a critical role in Rika's protection plan, stationed at her home during the festival to intercept attackers and coordinating countermeasures against the Yamainu paramilitary group. His karate skills enable him to defeat multiple assailants, including rival Tetsurou Okonogi, directly ensuring Rika's survival. Beyond main arcs, Akasaka appears in Tsumihoroboshi-hen's "The Demon's Script" TIP, investigating Hinamizawa's remains years after the disaster with Ooishi and colleague Sawada. They discuss links between the tragedy and Miyo Takano's research, later encountering an adult Rena Ryuugu. His relationships define him: devotion to Yukie and Miyuki shapes his motivations; friendship with Ooishi facilitates investigations; his bond with Rika evolves from skepticism to unwavering loyalty. After Yukie's death in most timelines, he raises Miyuki alone. Some comedic later manga interactions involve Rika jokingly calling him "daddy," confusing Yukie. Akasaka transitions from a somewhat naive investigator in 1978 to a resolute protector, driven by remorse and responsibility. Fragile moments surface, like in Nekodamashi-hen, where Hinamizawa Syndrome overcomes him, leading him to kill Rika before dying himself.