Mio Kofune is the younger daughter of the Kofune family and Shinpei Ajiro's adoptive sister, raised alongside him and her older sister Ushio on Hitogashima island. A first-year high school student, she has tanned skin, medium-length black hair, and blue eyes, typically dressed in a traditional Japanese sailor school uniform. Physically active, she belongs to the swimming club and excels in sports. Personality-wise, Mio exhibits a shy, reserved nature, contrasting sharply with Ushio's boldness, leading her to struggle with deep insecurities and self-hatred, feeling overshadowed. Despite this, she demonstrates kindness, protectiveness towards loved ones, and occasional cheerfulness, maintaining a relatively healthy attitude even after Ushio's death. She harbors unspoken romantic feelings for Shinpei, complicating their interactions. Her involvement in the island's shadow conflict starts when she confesses to Shinpei about witnessing Ushio's doppelgänger before Ushio drowned. In early timelines, she aids Shinpei's investigations but faces repeated tragedy: killed by her own shadow while seeking help in the first loop, and witnessing Sou's sacrificial death protecting her from a shadow attack at the festival in the third loop. Her shadow counterpart, created when scanned wearing her uniform, is emotionless, cold, and wields a kitchen knife as its weapon. As loops progress, Mio becomes integral to the group's resistance. In the sixth loop, she reconciles with Ushio's shadow after learning the truth and joins the fight. During the seventh loop, her shadow is captured and converted into an ally after Ushio pins it down for reprogramming. This converted shadow later protects Mio and urges her to confess her feelings to Shinpei, leading to a rooftop conversation where Mio plans to reveal her emotions. Tragedy recurs in the eighth loop when she and Sou are deceived and captured by a shadow impersonating Shinpei, resulting in their consumption by Haine. In the ninth loop, she avoids this fate after Shinpei and her shadow intercept them. She assists in safeguarding children during the final confrontation and navigates Sou's renewed confession, reaffirming her unchanged feelings while maintaining their friendship. Her development culminates in accepting her self-worth and resolving to express her emotions openly.