Kyōsuke Tamamo, also known as Tamamo, is a fox spirit (yōkai) roughly 400 years old at the start of the series, aging to 412 years later. Hailing from a lineage of high-ranking fox warriors, he first enters the human world as an antagonist. His specific goal is to acquire a perfectly fitting human skull to perfect his shape-shifting abilities, leading him to target Hiroshi Tateno due to the skull's compatibility. Disguised as a teacher intern, he attempts to kill Hiroshi but suffers defeat at the hands of Meisuke Nueno (Nube). Surviving this encounter, Tamamo becomes deeply fascinated by the surge of power Nube displayed during their battle, power Nube attributed to human emotions like love.

This defeat sparks a change. Tamamo shifts his focus from hostility towards humans to studying their emotions, particularly love, believing understanding it will grant him greater power to eventually surpass Nube. He adopts human guises, frequently as a surgeon or teacher, to observe and interact. His initial contempt for humanity gradually lessens. He repeatedly assists Nube and Class 5-3 against supernatural threats, even deploying his potent, self-sacrificing fire technique "Megido" to destroy Zekki, the younger brother of Nube's Oni no Te. Despite later opportunities to take Hiroshi's skull, he refrains, acknowledging his growing affinity for humans and the erosion of his original purpose. This hesitation creates internal conflict, as failure to secure a suitable skull risks his mental and physical deterioration.

His evolution continues in spin-offs and sequels. In *Reibaishi Izuna: The Spiritual Medium*, he encounters spiritual medium Izuna Hazuki, who initially distrusts him as a threat. Though he considers eliminating her, he spares Izuna after witnessing her compassion for children. Years later, in *Jigoku Sensei Nube Neo*, he declines Nube's request for aid against the antagonist Daira Setsuki Tentei, citing injuries to the Kyuubi (Nine-Tailed Fox) under his care. He reappears near the saga's conclusion, expressing relief at Nube's success in averting global catastrophe. His longevity and adaptability are evident through his presence across timelines, including brief appearances in the final chapter of the original manga and the *Izuna* special.

Physically, Tamamo typically manifests as a handsome, long-haired man tied with a band. His attire often consists of a blackish-green suit with a white or green jacket. Releasing his hair band signifies unleashing his full power. His true form is a fox-like beast man standing 6.2 feet tall, wearing a green mantle. His hair color varies: pale blonde in the manga, OVAs, and 2025 anime, but silver in the 1996 anime.

Tamamo wields centuries-honed spiritual abilities. He masters "illusion jutsu," manipulating perceptions through motion, such as creating clones via video screens. Offensively, his "yōko foxfire jutsu" generates and controls intense flames. His primary weapon is a neck catcher resembling a paw, designed for skull extraction but also usable as a combat barrier. He can assist in controlling spiritual energy and perform techniques like "youshin jutsu." His most devastating ability, "Megido," requires self-sacrifice and is reserved for dire threats.

His relationships mirror his transformation. Initially viewing Nube solely as a rival to defeat, he develops unspoken admiration, avoiding direct confrontations and saving Nube's life multiple times. He ultimately abandons his quest for Hiroshi's skull, symbolizing his embrace of human connections. Though initially antagonistic toward Izuna Hazuki, he spares her after recognizing her humanity. A trivia note states his name alludes to Tamamo no Mae, a legendary nine-tailed fox, though later lore reveals the actual Nine-Tailed Fox disdains him, making the connection honorary or aspirational.