Gyuki, originally a human noble named Umewakamaru, has a tragic origin that explains his fierce loyalty and cautious nature. Born into a house of nobles, Umewakamaru was sent at the age of seven to a temple on Mount Hiei to pray for his deceased father's soul. There, his aptitude for studies drew jealousy from his peers, who physically injured him. Feeling alienated and missing his mother, who had left him at the temple, he snuck out to find her in the capital. On his journey, he was deceived by two yokai at Otsu who claimed his mother was ill. They led him to Mount Nejireme, where he discovered that the same yokai had already devoured his mother. Umewakamaru was then eaten himself. However, through the sheer force of his hatred and will, he was reborn as a powerful yokai, destroying the creature that had killed his mother and himself in the process and eventually taking on its name, Gyuki.

In terms of personality, Gyuki is renowned for being a careful and meticulous planner. He is frequently described as calm and quiet, a figure who listens to all available facts before passing judgment, and his opinion is highly regarded within the Nura Clan. Once he reaches a conclusion, he commits to seeing it through to the end, and his strategies are consistently made with the Nura Clan's best interests at heart. His intense loyalty stems from a deep appreciation for those who acknowledge his strength and worth, a trait that defines his most important relationships.

Gyuki's primary role within the narrative is as a high-ranking board member of the Nura Clan and the chairman of his own Gyuki Clan. He initially challenges Rikuo Nura, the young heir to the clan, to test his fitness to be the next Supreme Commander. Luring Rikuo to Mount Nejireme, Gyuki orchestrates a conflict to see if the boy possesses the necessary strength and will. When Rikuo proves himself and acknowledges Gyuki's true intentions, Gyuki accepts him as a worthy leader and even attempts an honorable suicide to atone for his disloyalty, though Rikuo stops him. After this arc, Gyuki becomes one of Rikuo's most steadfast and crucial supporters. During the Kyoto arc, he plays a vital role by arriving to save a severely injured Rikuo from the Tsuchigumo. He then takes Rikuo to Mount Kurama to recover and undergo intense training, instructing him that to become stronger, he must embrace his human side and surpass the Fear of his grandfather, Nurarihyon.

Key relationships define Gyuki's actions and development. His unwavering respect for Nurarihyon, the current Supreme Commander, is absolute. In the past, Gyuki had built his own clan into a formidable force, but after a battle with the Nura Clan, Nurarihyon was so impressed that he requested Gyuki join his Hyakki Yakō (night parade of a hundred demons). Gyuki's loyalty to Nurarihyon is the foundation of his commitment to the clan's well-being. His relationship with Rikuo evolves significantly over the course of the story. Beginning as an adversary who tests the young heir, Gyuki transitions into a mentor and protector. He is shown to have played with Rikuo when the latter was a child, demonstrating a long-standing connection. After Rikuo shows mercy and understanding, Gyuki transfers his fierce dedication to the young master, helping him unlock new levels of power.

His notable abilities include a formidable technique known as Hallucination. This power projects visions of the past, causing the enemies defeated by Gyuki to appear one after another, which erodes the opponent's will to fight. He can also control these illusions to speak his thoughts directly to his targets. Physically, he stands at a notable 185 cm tall and possesses a muscular build. He typically appears with dark green hair that often covers half of his face and a short beard, usually dressed in traditional yokai attire.