When Gyūki was still human, he was called "Umewakamaru". When he was 7 years old, he went into service on Mt. Hiei to pray the salvation of his fathers soul and that was also his final farewell to his mother, which he and his mother where been seen talking with each other. He quickly surpasses the seniors and on age 10, his attitude had become well known on Mt. Hiei, but on the same time, it also formed a seed of jealousy among his fellow students. When he was 12 his eye got injured three times from stones that come out of nowhere. He then taught the doesn't belong at the temple and misses his mother because she is the only one who understand and loves him. He then sneak out of the temple to look for her. Umewakamaru reached Lake Biwa, where he is encountered by two unknown womens using cunning words to kidnap Umewakamaru. They knew his name and says that his mother is ill, saying that she has been collapsed at Lake Biwa and that they brought their at their mansion and ask him to come with them. Umewakamaru then follows them, but they brought him to Mt. Nejireme that was the location where the frightful "Gyūki" appears. He attacks Umewakamaru which he says that want not to die before he see his mother and that he have to survive. He then shows Umewakamaru his mother, which he goes inside his mouth, when he tries to rescue her, his will overcame his spiritually and transform into a demon. The demon "Gyūki" has been born, holding his mother corpse. He then gets of the yōkai, which he starts to attack humans. He piled up corpses as an offering for the salvation of his mother. He gathered yōkai's and attacked human dwellings. Later he became to known as the yōkai, Gyūki. Years has been passed and he forgot about his mother love. The first generation Nura Clan has been arrived and challenged Gyūki and his servants without hesitation. After 3 days and 3 nights the battle has been settled and the one who has won the battle is the Nura Clan. Nurarihyon then says that he is strong just like the rumors said and ask him to become his comrade. Gyūki knew later that Nurarihyon was testing him. Later they where been seen exchanging Sakazuki. Gyūki never will forget the words what Nurarihyon said to him when they where been exchanging Sakazuki. He then slashed Rikuo, he then tells Rikuo that he must take the path of evil and that he must surpass him, which Rikuo has made his final blow against Gyūki.
- Gozumaru and Mezumaru are both shown as members of Gyūki's clan before it became a part of the Nura Clan, making them both at least 400 years old.