Seidō Hishigata directs the sole hospital on Hitogashima Island, Hishigata Clinic. He is the father of Sou and Tokiko Hishigata. Driven to save his terminally ill wife, Chitose, he struck a bargain with the shadows: he would turn Chitose into a shadow and provide corpses from his clinic as sustenance for Haine, the mother of shadows. He believed that after Haine consumed her "final meal" at the summer festival, Chitose would return, and his family would gain eternal life as shadows within Haine's world. The Hishigata family has served the shadows for over three centuries, with only one successor chosen each generation. Seidō selected Tokiko as his heir, deeming the role too heavy for Sou, and kept his shadow dealings secret from his son. Seidō supplied human bodies for the shadows, including Shinpei Ajiro's parents, killed by Shide to prevent their discovery of Hiruko Cave; records of these victims were hidden in a safe. During the time loops, Seidō coordinated shadow activities, such as instructing Negoro to deliver a policeman's body for Haine. His clinic concealed a secret passage to Hiruko Cave, unlocked by a key hidden inside a Hiruko statue, later found by Shinpei, Ushio, and Sou. In the seventh loop, the shadows, deciding Seidō was expendable, sent Chitose's shadow to strangle him. Rescued by Shinpei's group and revived via CPR by Sou, Seidō disarmed his son and shot Tokiko—later revealed as Shadow Mio in disguise. He initially resisted cooperation, demanding Tokiko kill the group to regain Haine's trust. Learning of the shadows' betrayal and plan to abandon his family shattered him. Confronted by Chitose urging atonement, Seidō confessed his desire for eternal shadow life for his family. He then provided evidence against the shadows: the hidden victim list, Shide's true identity as Shidehiko Hishigata (the clinic's founder and Haine's first shadow offspring), Chitose's true death date, and that he had preserved her ashes instead of offering them to Haine. Seidō possesses immunity to shadow copying, a result of prior scanning, which prevented Shadow Mio from accessing his memories. In the ninth loop, he cooperated with Shinpei regarding Hizuru Minakata's body and discussed countermeasures against Shide.