Kuromaru emerges as a solitary figure shaped by isolation and torment, driven by bullying to forge an alliance with the enigmatic Iron Maiden—a choice that entwines his fate with Gabi, a faction resisting the predatory Busoma demons. Born from the forbidden Zanza ritual, which fuses the essence of slain demon beasts to a human host, he exists as a sentient demon sword pulsating with raw power. Though the specifics of his origins linger in shadow, his existence hinges on this union of human vessel and demonic energy, designed to function as a living weapon.

Bound initially to the Sword Administration Bureau, he channels unwavering loyalty toward his host, Amon, adhering rigidly to duty and protocol. Proximity to humanity, however, sparks a metamorphosis: he grapples with existential doubts, evolving from an unflinching instrument of war into a being capable of empathy. This awakening propels him into direct confrontation with the Shoshidai, whose ambitions to weaponize demon swords threaten global subjugation.

Central to his journey is Gai Ogata, whose intervention during an assault by Gabi operative Kazumo forges a partnership anchored in trust. Under Gabi’s guidance, Kuromaru embraces renewal—receiving a prosthetic arm, pursuing meditation, and mediating alliances between factions. His pragmatic, methodical nature clashes with a growing aversion to blind obedience, yet his analytical mind becomes an asset in navigating moral complexities. Once defined by subservience, he gradually reclaims agency, balancing loyalty with introspection to aid resistance efforts against apocalyptic threats. His arc traces a path from hardened enforcer to a pivotal figure reconciling duty with the fragile nuances of humanity.