Misaki Sakimiya witnessed a serial killer murder her parents during her early childhood, leaving her emotionally scarred and fixated on the killer's perceived enjoyment of murder. This trauma made her question if killing could bring pleasure. After placement in an orphanage, she confessed to visitor Lisa "Clarissa" Kuraki that she had forgotten how to smile and sought to relearn by mimicking others' apparent joys. She concluded that "killing people" was the answer, drawing from her traumatic memory. As a teenager, Misaki became the assassin Zaki, specializing in eliminating other assassins and mobsters, earning a "killer of killers" reputation under Clarissa's agency Youtoukorou. She accepted Clarissa's personal contract to assassinate her parents' murderer. During their confrontation, Misaki credited him for inspiring her path, denying her actions were revenge or justice while interrogating his enjoyment of killing. Her pivotal assignment was the contract killing of innocent sixteen-year-old Polka Shinoyama, a test of her capacity to murder non-combatants. She slit his throat with a prybar in Shinjuku. After deeming him dead, she attempted suicide by jumping off a building but aborted when associate Takumi Kuruya revealed Polka had survived. Tracking him to an abandoned building, she chased Polka only to be impaled through the stomach by a massive skeletal hand summoned by the entity possessing Polka's body—the necromancer Corpse God—resulting in her death. The Corpse God resurrected Misaki as a zombie, restoring her body and granting supernatural abilities witnessed by Youtoukorou personnel including Clarissa. Clarissa assigned Misaki and Takumi to assist the Corpse God's adaptation to Earth. As a zombie, Misaki gained enhanced strength, rapid regeneration healing injuries like bullet wounds, and paralytic saliva immobilizing targets via bites. Her form later evolved into a Vampire Larva, growing bat wings for flight and upper fangs. She retained childlike innocence and fascination with death but developed fierce loyalty to the Corpse God and allies. She aided his fortunetelling business and participated in underworld activities, such as a yakuza conflict where she instantly regenerated after being shot. During this incident, the Corpse God subdued attackers by knotting their limbs—an event dubbed the "human knots" incident. Her bond with the Corpse God deepened through supporting his endeavors, showing genuine concern for his well-being and goals. She formed a pragmatic partnership with Takumi, acting as their trio's physical enforcer while relying on his technological expertise for reconnaissance. Despite her violent history, she demonstrated compassion toward children and those she considered friends.