Gyuki, originally human and named Umewakamaru, was born into nobility. His father's death at age seven led him to a Mount Hiei temple to pray for his father's soul. There, his scholarly aptitude provoked jealousy among peers, resulting in repeated injuries. Learning of his mother's illness, he left the temple seeking her but was deceived by Ootsu yokai who claimed she resided on Mount Nejireme. At the mountain, he discovered the original Gyuki yokai had consumed his mother; the same creature then devoured Umewakamaru. Consumed by hatred and will, he resurrected as a yokai, destroying the original Gyuki and inheriting its name and form. As a yokai, Gyuki founded a powerful warrior clan that clashed with the Nura Clan. Following defeat, Nurarihyon acknowledged his strength and invited him into the Hyakki Yakō. Gyuki later joined Nurarihyon's raid on Osaka Castle to rescue Yōhime, battling the Great Tengu of Mount Kurama despite warnings about the mission's peril. He also interacted with the young Rikuo. Gyuki serves as a high-ranking Nura Clan board member and chairman of his own clan. He possesses a muscular build, dark green hair covering the right half of his face, and jawline stubble. He typically wears a purple shihakusho lined with white and a green haori, standing 185 cm tall and weighing 83 kg. His demeanor is calm and analytical. He meticulously strategizes, prioritizing the Nura Clan's interests and evaluating all facts before judgment. His loyalty stems from appreciation for those who acknowledge him, extending first to Nurarihyon and later to Rikuo. He masterminded a plot with Hitotsume Nyūdō and Sorobanbō to test Rikuo's suitability as Third Heir, luring him to Mount Nejireme. After Rikuo demonstrated leadership, Gyuki acknowledged him and attempted suicide for his disloyalty. Rikuo intervened, later pardoning him at a clan meeting. In subsequent conflicts, including the Shikoku and Kyoto arcs, Gyuki advised on countermeasures against threats. He facilitated Rikuo's recovery and training after the Tsuchigumo attack, negotiated with the Great Tengu of Mount Kurama for Rikuo's training, and witnessed Rikuo's first use of the matoi technique. Gyuki's abilities include inducing hallucinations of an enemy's past victims to erode their will to fight, projecting controlled messages through these illusions. His key relationships involve unwavering respect for Nurarihyon and evolving loyalty to Rikuo, transitioning from initial opposition to steadfast support.