Zero is the protagonist in post-apocalyptic dystopian Tokyo (AD 2917), where demonic invasions push humanity toward extinction. He is the first subject and prototype of the Ragnarok Institute's "Killer Project," designed to create genetically enhanced super-soldiers called "Killer Princes and Princesses." These warriors wield specialized weapons to combat the demons. His designation "Zero" marks his status as the program's foundational subject.

Forged through brutal genetic mutation experiments, Zero gains enhanced combat abilities vital for facing demonic threats, but this also renders him extremely vulnerable under the institute's control. His existence is defined by the conflict between his engineered purpose as a weapon and his struggle to retain humanity against the institute's dehumanizing protocols. The narrative follows his efforts to navigate the human world, seeking resolution to the demonic crisis while grappling with the ethical and existential ramifications of his creation.

The anime concept film depicts Zero actively rebelling against the Ragnarok Institute's authority. This defiance is driven by his refusal to be reduced to a mere instrument, particularly when witnessing the institute's treatment of another subject—a "Killer Princess" linked to a legendary weapon. Their shared experience of exploitation becomes a catalyst for his resistance. His trajectory is one of rejecting his creators and his predetermined fate as a disposable military asset, though long-term outcomes for his arc remain unexplored.