Isamu is one of three knight dolls forged to protect the royalty of the Doll Kingdom. Initially bound by a bracelet and amulet to serve Princess Yae, his loyalty shifted when Yae, falling in love with a human, transferred the bracelet to Rui's grandfather for his protection. After Yae became possessed by the demon Devaul and orchestrated the bracelet's theft from Rui, Isamu was forced to serve Scarecrow and Yae through the amulet Yae retained. During this period, he acted as an antagonist under the belief he was upholding his duty to the kingdom, functioning as a reluctant minion. His defiance of Yae's orders led to his deactivation.

Rescued by Doll Licca, Isamu abandoned his service to the villains and realigned with the protagonists. He became permanently bonded to Dai Takabayashi, a classmate of Licca Kayama, after Dai received the blue bracelet known as the Calling Watch. This artifact allows Dai to summon Isamu using the incantation "Pereso Perese Burune Buraine". As a guardian, Isamu wields the large sword Light Thunder, projecting energy blasts and cutting through adversaries primarily through ranged combat. He is distinguished as the sole male member among the Doll Knights. Visually, he is a blue-haired figure in heroic attire, focused on protecting Dai and assisting Licca's group against threats from the Doll Kingdom.