Hōō Onigashima reigns as Shishiku Academy's strongest third-year bancho, a "legendary delinquent" famed for unmatched combat skills and judo mastery. He shares a father with half-siblings Hikaru and Hinako Onigashima but carries undisclosed maternal heritage. Adopted after his biological father Kaizu Shinya perished defending Hikaru's uncle, Hōō fiercely upholds the Onigashima legacy. This loyalty fuels protective devotion toward his siblings, though Hikaru's open resentment fractures their bond. Hōō warmly supports Hinako—initially oblivious to her impersonation of Hikaru at Shishiku—relishing when she calls him "onii-san" and urging her to lean on him.

When rival yakuza faction Urashima threatens the family, Hōō's resolve hardens. Discovering Hinako's biological sex while nursing her through illness sparks a confrontation where she reveals the twin deception. Though enraged by Hikaru's deceit, Hōō agrees to conceal Hinako's identity from their father. To prevent family strife, he claims the Onigashima succession with a contingency: renouncing the name should Hikaru become heir, acknowledging his non-biological status while shielding his siblings.

Amid escalating Urashima hostilities, Hōō battles Kenji—his uncle and the Onigashima's sworn enemy—to defend Hinako. He nearly slays Kenji until Hinako intervenes, cementing his commitment to her principles. Post-conflict, Hōō confesses romantic feelings and proposes. Their marriage earns paternal approval contingent on Hōō formally joining the family, culminating in a traditional wedding where he vows unwavering devotion. As a husband, he displays playful jealousy over Hinako's school friendships yet prioritizes her well-being and family unity above all.

Hōō evolves from revered academy leader to self-sacrificing heir, embodying familial duty and loyalty. He stands 180 cm tall with claret hair swept back, crimson eyes, and a signature style blending gakuran uniforms with coat-cape attire.