Sou Hishigata, a 17-year-old resident of Hitogashima Island, is the eldest son of the family running the island's sole medical facility, Hishigata Clinic. He aspires to become a doctor, driven by a strong sense of justice and dedication to preserving human life, a motivation fueled by profound regret over witnessing but being unable to prevent the drowning death of his childhood friend Ushio Kofune. He repeatedly apologizes for this to her adoptive brother, Shinpei Ajiro. Sou harbors longstanding romantic feelings for Ushio's younger sister, Mio Kofune, having confessed twice and been rejected both times. Physically, Sou has messy brown hair and brown eyes. His personality blends compassion with impulsiveness, often prioritizing others' safety over his own. In early timeline loops, he alerts Shinpei to critical details about Ushio's death, including strangulation marks suggesting murder, and shares rumors about shadows copying residents, aiding in unraveling the island's mysteries. Across multiple timeline resets caused by Shinpei's looping ability, Sou's involvement intensifies. During the third loop's festival, he attempts another confession to Mio. When a shadow version of himself attacks her with an axe, he sacrifices himself by shielding her, resulting in his death, underscoring his protective nature and unresolved feelings. In the fourth loop, after learning about Ushio's sentient shadow from Shinpei, Sou assists in investigating the old Hishigata Clinic. He recalls his father preventing him from entering as a child, hinting at hidden family secrets. Within the clinic's tunnels, the group encounters defective shadows and confronts Sou's sister, Tokiko. She reveals the Hishigata family's generations-long servitude to the shadow entity Hiruko (Haine), explaining their mother was transformed into a shadow to prolong her life, a plan requiring Hiruko's survival. When a shadow shoots at Tokiko, Sou intercepts the bullet, sustaining a severe injury. Later loops unveil further familial revelations: Sou's father, Seido, collaborated with Hiruko to turn the family into shadows for interdimensional travel, exposing centuries of manipulation by Shidehiko Hishigata, the clan's shadow-founder. Sou participates in the final battle against the shadows, wielding weapons like a baseball bat and specialized nail gun. His medical skills prove vital, such as administering emergency aid to his father after a shadow attack while insisting Seido must answer for his actions. Sou's development culminates in his acceptance of the family's dark legacy and unwavering commitment to protecting islanders. Despite the trauma of discovering his mother's fate and his father's betrayals, he remains a steadfast ally against the shadows, leveraging both combat abilities and medical knowledge throughout the conflict.