Tokiko Hishigata, younger sister to Sou Hishigata, is a first-year high school student on Hitogashima Island. Academically gifted and consistently achieving top grades, she struggles physically and lacks skill in sports. Her closest friend is Mio Kofune, a classmate since childhood. Her family runs the Hishigata Clinic, the island's sole hospital. Tokiko becomes entangled in the supernatural conflict with "Shadows"—entities that copy and replace humans—due to her family's secret, generational service to Hiruko (Haine), the mother of Shadows. This service involves supplying deceased humans for Haine's consumption. Tokiko collaborates with the Shadows believing Haine's survival is essential to sustain her mother, who Haine transformed into a Shadow to save from a fatal illness. This belief drives her initial antagonism, including aiding in the capture of Ushio Kofune's Shadow. During the third loop's festival, Tokiko helps Mio dress in a yukata and joins the group. She reveals knowledge of Sou's feelings for Mio and indirectly warns Shinpei that Mio loves another. Shadows attack, and Tokiko meets her death while apologizing for her actions. In the fourth loop, she confronts Shinpei, Ushio, and Sou in the underground caves, commanding two baby-like Shadows. She discloses her family's allegiance to Haine and justifies her collaboration by her mother's dependence on Haine. When a Shadow betrays their agreement, taking Mio hostage and declaring intent to exterminate humans during the festival, Tokiko protests. The Shadow shoots her, but Sou intercepts the bullet. Tokiko is then shot and killed. After multiple loops, Tokiko learns the Shadows plan to kill her and have deceived her family, prompting her to align with Shinpei's group. She joins the resistance, utilizing her control over two stray Shadows named Rosencrantz and Guildenstern in battles. She allows Shadow Mio to copy her memories and feelings to aid the group. In the seventh loop, she helps subdue Shadow Mio and later provides first aid to her father, Seido Hishigata, after an attack. Seido shoots her in retaliation for her defiance; she survives temporarily due to Shadow Mio's intervention. In the eighth loop, Tokiko fights alongside Hizuru Minakata against Shidehiko Hishigata. Fatally injured, she implores Shinpei to save Hizuru and Ryunosuke before dying. Shadow Mio scans her memories, revealing her experiences to Shinpei. During the tenth loop, Tokiko remains in the Hiruko Cave to fend off an attacking Shadow while the group advances toward Hiruko's main body, sacrificing herself to ensure their progress. Her character arc concludes with her recognizing the Shadows' betrayal and dedicating her final efforts to protecting others, expressing particular concern for her brother Sou's well-being in her last moments.