Masao Horiki emerges as a shadowy operative resisting the S.H.E.L.L. system—a corporate-engineered network using nanotechnology to abolish mortality while perpetuating social divides. Hailing from the margins of a deprived underzone, he operates as both a drug-dealing gang enforcer and strategic advisor to Takeichi, leader of a rogue biker faction assaulting the privileged "Inside" enclave. His crusade stems from visceral opposition to S.H.E.L.L.'s dominion over human existence, advocating systemic annihilation to forge a new order where mortality’s return might reignite life’s intrinsic value.

Among the first-generation "applicants" gifted with rare command over the Lost—mutants severed from S.H.E.L.L.'s networks—he weaponizes narcotics to trigger forced metamorphoses in others, harvesting their destabilizing power for his insurgency. His machinations pivot on Yozo Oba, whose transformation into a hybrid Lost-human he engineers to weaponize the youth’s latent potential for reshaping Japan’s trajectory. This scheme mirrors his conviction that humanity’s deathless stagnation demands violent renewal.

Once a physician displaced by S.H.E.L.L.’s automated medical supremacy, his disillusionment fuels an anarchist philosophy positioning him as the "destruction" vector within the Civilization Bringing Curve, a predictive framework for civilizational trajectories. This ideological antithesis to Yoshiko Hiiragi’s "restoration" approach erupts in a climactic showdown where Horiki unveils his endgame: leveraging Yozo’s hybridity to fracture S.H.E.L.L.’s hegemony.

His dynamic with Yozo oscillates between tactical guidance and cold exploitation, preying on the younger man’s existential fissures to advance his vision. Though framed as an antagonist, his motivations anchor in restoring unmediated human agency, rendering him a morally ambiguous force within the narrative’s dissection of technocratic control, mortality’s erosion, and societal rot.