Warrius Zero commands the Earth fleet's battleship Karyu in the aftermath of a catastrophic war between humanity and mechanized beings. His wife and child died by the hands of machine men, forging his motivations through profound personal loss. He upholds duty under a fragile post-war peace favoring mechanized forces, leading a mixed crew of humans and mechanized soldiers plagued by tension and prejudice. Zero enforces strict equality, compelling all to function solely as soldiers while suppressing his deep-seated resentment toward mechanized beings.

His primary mission targets space pirate Captain Harlock, once an ally, now an adversary driven by duty—though a complex mutual respect emerges between them. Aboard the Karyu, interactions with mechanized first officer Marina Oki challenge his worldview. Marina, an aquatic humanoid transformed to survive her planet’s ecological collapse, sparks romantic feelings that complicate his prejudice and fuel internal conflict.

Evidence reveals the peace as a facade, exposing mechanized schemes for dominance. This forces Zero to reassess his loyalties. Confrontations escalate into large-scale battles where the Karyu allies with pirate factions, including Harlock’s forces, defending planets like Technologica against mechanized threats such as the Helcastle fortress and its commander, Zeth Voder.

In the OVA specials "Hunt for Young Harlock," pirates capture Zero. After escaping with unexpected aid, he duels Harlock—only to form a temporary alliance against mutual enemies. During this conflict, he encounters dimensional portal technology guarded by Harlock and Tochiro; its full significance remains unresolved.

Zero’s arc shifts him from a grief-stricken, duty-bound officer bound by prejudice toward a leader capable of defying unjust authority. He forges alliances through mutual respect with former adversaries like Harlock and mechanized individuals like Marina, transformed by experiences challenging his beliefs about mechanized beings and the post-war power structure.