Doll Isamu, one of three knight dolls originally charged with protecting the Doll Kingdom's royal family, was bound as a servant to Yae through possession of his bracelet and amulet. When Yae developed romantic feelings for Rui's grandfather, she transferred the bracelet to him, binding Isamu as his protector.

After Yae became possessed by the demon Devaul, she commanded Scarecrow to steal the bracelet from Rui. With Yae retaining the amulet, Isamu was forced to obey her, serving as Scarecrow's bodyguard and aiding Yae's schemes under antagonistic control.

Isamu ultimately defied Yae's orders, resulting in his deactivation. Following rescue by allies, he transitioned to a heroic role. Rui rediscovered the magical incantation "Pereso Perese Burune Buraine" required for his summoning using the Calling Watch, enabling Isamu to become the dedicated protector of Dai, who received the associated blue bracelet.

In combat, Isamu wields the massive sword "Light Thunder," projecting energy attacks and slicing enemies. He functions alongside Doll Licca and Doll Izumi as one of the trio known as the Doll Knights, deploying his abilities to defend Dai and Licca's allies against threats from the Doll Kingdom.