Hayate Kirino, orphaned in childhood, was raised with Kagemaru “Kabamaru” Igano under the strict tutelage of their adoptive grandfather, Saizo Igano—a ninja master who drilled both boys in rigorous physical disciplines. Their shared desperation to escape Saizo’s harsh methods led Hayate to suggest they split up during a failed escape attempt. While Kabamaru endured punishment for shielding Hayate’s location, Hayate evaded capture, haunted by enduring guilt over abandoning his brother-in-arms.

Years after Saizo’s apparent death, Hayate emerged in Tokyo as an operative for Suu Matsuno, headmistress of Oogyoku School and rival to Kabamaru’s mentor, Ran Ookubo. Tasked with undermining Kabamaru and Kin'gyoku School, Hayate executed schemes that included sabotaging Kabamaru’s relay race with a shuriken strike and conspiring with Kin'gyoku student Shirakawa to drug him. These acts reflected Hayate’s torn allegiance between his employer and lingering loyalty to Kabamaru.

A turning point arose when Ran inadvertently consumed the drugged candy meant for Kabamaru, prompting Hayate to rescue her from an oncoming vehicle—exposing his fractured moral compass. Later, during a school trip to Iga’s mountains, Hayate confronted Kabamaru alone, confessing he’d manipulated their bond to flee Saizo’s tyranny and urging Kabamaru to cut ties. Yet when a snowstorm endangered Kabamaru, Hayate secretly ensured his safety, betraying lingering camaraderie.

The schools’ escalating rivalry climaxed in a duel where Hayate, defeated and submerged in water, was presumed dead. Saizo’s sudden return—revealing his death as staged—catalyzed reconciliation. Wounded but repentant, Hayate admitted his failures and rejoined Saizo and Kabamaru in the mountains, resuming their ninja training and marking his redemption through renewed dedication to their shared legacy. His arc traced a journey from guilt-driven fugitive to a man reconciling duty and brotherhood, ultimately reclaiming his place within the bonds he once strained.