Sou Hishigata, Shinpei Ajiro's 17-year-old childhood best friend, lives on Hitogashima Island. As the eldest son of the Hishigata family, who run the island's only hospital, Sou trains to become a doctor. He carries deep guilt for witnessing Ushio Kofune drown in a maritime accident, regretting his inability to intervene despite being present. Sou confessed romantic feelings to Mio Kofune twice, rejected both times. Possessing a strong sense of justice and protective instincts, Sou sacrificed himself during the third time loop, shielding Mio from an axe attack by his own shadow at a summer festival. Learning of the shadows' existence from Shinpei and Ushio in later loops, Sou joined their investigation. He aided in exploring the Hiruko Cave, utilizing baseball gear for defense and a nail gun as a weapon against shadow entities. His medical knowledge and analytical skills contributed to strategies, including creating a code-word system to identify shadows. Sou's family became central to the conflict. His father, Seido Hishigata, collaborated with the shadow Haine to transform Sou's ailing mother into a shadow. Confronting his father over this betrayal, Sou revived Seido using CPR after a suicide attempt. In the final confrontation, Sou took a bullet meant for his sister Tokiko. Across multiple loops, Sou consistently allied with Shinpei and Ushio. In the sixth loop, witnessing Ushio's memories of previous timelines cemented his commitment. By the seventh loop, he participated in battles against Haine and Shide, contributing to their defeat and the island's survival. Sou survived the final timeline. His relationship with Mio remained unresolved, though he persisted in expressing his feelings for her.