Takeshi Onimaru emerges as a formidable high school kendo prodigy locked in a fierce rivalry with Yaiba Kurogane after their unresolved first duel. Driven to dominate, he unearths the ancestral Fuujin Sword, tied to the wind deity Fuujin, within his family’s concealed basement. The sword’s influence twists him into a demonic being marked by horns, heightened strength, and a merciless drive for global conquest. His monstrous visage boasts intricate armor adorned with skull designs, jagged shoulder spikes, and a jetpack, emblems of his dark supremacy. Initially defined by harsh discipline, quick temper, and vengeance, Onimaru harshly disciplines followers who falter. He commands dark magic to conjure fiendish servants, ensnare foes through hypnosis, and revive legendary fighters, wielding these forces to enforce his rule. Signature techniques like the Fuujin Ha, unleashing razor-edged wind slashes, and the Tatsumaki Giri, generating devastating tornadoes, grow increasingly catastrophic as his power surges. Collaborating with Professor Kanabo, he integrates advanced arms like the Fuurai Bazooka into his arsenal, reflecting his calculated brutality. A pivotal shift arises during the Kaguya crisis, compelling Onimaru into an uneasy partnership with Yaiba to thwart lunar princess Kaguya’s cosmic menace. Their alliance peaks in the Furaijinha maneuver, fusing their blades’ elemental energies to vanquish a galactic foe. When Yaiba destroys his demonic horns, Onimaru regains humanity, abandoning tyranny. Redeemed, he trains under Yaiba’s father, Kenjurou, embracing tranquility and redirecting his drive toward principled martial excellence over domination. Beyond canon, as seen in the *Detective Conan OVA: Conan vs. Kid vs. Yaiba*, Onimaru persists as a demonic figure, dispatching minions to ambush Yaiba and Kaitou Kid mid-air. Conversely, within *Detective Conan*’s primary continuity, he diverges as a composed, ethics-bound kendo master stripped of supernatural ties. This iteration upholds samurai honor, declining accolades from humility and projecting quiet dignity. His journey spans antagonist to anti-hero, grappling with corruption, redemption, and power’s dual nature. Central bonds include his evolving rivalry-to-respect rapport with Yaiba, the calculated dominance over henchmen like Gerozaemon Geroda, and Kenjurou’s guidance steering his ethical rebirth. Armaments like the Devil King Sword and Fuujin Sword mirror his shifting loyalties, while his ultimate renunciation of demonic forces underscores the tension between aspiration and honor.