Kouta Oyamada, an orphan raised by his grandfather in the countryside after his parents' early death, transfers to Kunpō High School in the city. A childhood near-drowning left him with a lasting fear of swimming. Physically, he is short with brown hair and striking silver eyes. His mild, shy nature often manifests as indecisiveness, particularly when facing aggressive romantic attention. At school, frequent public compromising situations—often initiated by others despite his reluctance—earn him nicknames like "Great Erotic King" or "beast."

His life transforms upon meeting Chizuru Minamoto, a second-year kitsune (fox spirit) who declares her love. Though initially embarrassed by her overtly sexual advances, he develops genuine feelings. Their profound bond enables them to fuse into a powerful black-furred, five-tailed fox spirit, granting Kouta temporary supernatural abilities like enhanced speed and "fox flame" attacks.

Manga-exclusive lore reveals Kouta was artificially created by the organization "Kazu no Ha" (葛の葉) as a "divine vessel" for the god Susanoo-no-Mikoto. He vanished with his parents under unclear circumstances before being raised anonymously by his grandfather. His instant connection with Chizuru stems from her being the reincarnation of Kushinada-hime, Susanoo's wife.

His relationship with wolf spirit Nozomu Ezomori begins with helping her in a mundane dispute and evolves into her becoming his self-declared mistress, creating a love triangle. While the anime depicts him as persistently flustered by both women, the manga shows greater development: he eventually accepts Nozomu as part of a polyamorous relationship alongside Chizuru, formalizing a "marry them all" dynamic.

Character development intensifies in the manga. He transitions from reluctance to proactive commitment, openly declaring his love for Chizuru on the bridge where they first met. He matures to embrace his role as her future husband, later marrying her and fathering their biological daughter. They also adopt twin spirit hunters, Ren and Ai Nanao, raising them as daughters.

Repeated fusions with Chizuru augment his magical power, enabling him to detect other youkai and wield stronger fox flames. However, these transformations gradually and permanently alter him into a youkai. Wearing a scarf infused with Chizuru's essence allows him to assume a weaker fox form independently.

He faces traumatic experiences, including school ostracization due to his relationships with youkai and life-threatening confrontations—such as an attack by sickle weasel spirit Omi Kiriyama and conflicts with Nozomu's brother Saku, who places a lethal seal on him to inhibit Chizuru's proximity.