Boneco Isamu is one of the three Doll Knights guarding the royal family of the Doll Kingdom. Initially manipulated by the antagonist Devaul, possessing Yae Kayama, he serves as bodyguard to the villainous Scarecrow, believing his actions benefit the kingdom due to Devaul's influence and the theft of his summoning artifact, the Calling Watch.

He wields the large sword Light Thunder, capable of firing energy projectiles and cutting through enemies. Under Devaul's control, he opposes the protagonists as an Anti-Villain, lacking true malice despite his actions. A pivotal moment occurs when he refuses Devaul's command, leading to his capture. Rescued by Boneco Licca, he undergoes a Heel–Face Turn and joins the protagonists.

Following his redemption, Isamu becomes a dedicated protector of Dai Takabayashi, a classmate of Licca Kayama. Dai inherits the Calling Watch, summoning Isamu with the spell "Pereso Perese Burune Buraine." Post-transformation, his heroic nature is reflected in a blue color scheme. His combat style emphasizes strength and precision, delivering decisive blows against threats to Licca and her allies.

His core arc, transitioning from an unwilling minion to a loyal guardian, remains consistent across all official media. As the sole male Doll Knight, he holds a distinct position within the trio.