Gyūki, also known as the Eight-Tailed Beast or the Eight-Tails, is the second strongest of the Tailed Beasts in Naruto, behind Kurama, and a supporting character in Naurto: Shippūden. Gyūki was a destructive adversary in the history of Kumogakure, before being befriended by Killer B, the current jinchūriki of it.
History[]

Hagoromo talking to the young tailed beasts
After the Ten-Tails was defeated by Hagoromo and Hamura Ōtsutsuki, the former became the Ten-Tails jinchuuriki and the two of them used Chibaku Tensei to create the moon out of its husk. In order to prevent the Ten-Tails from rampaging again after his death, Hagoromo used the Creation of All Things Technique to divide the Ten-Tails into nine separate beings, each with a differing number of tails corresponding to their strength. These monsters would become known to humanity as the Tailed Beasts, and despite Hagoromo and his followers building temples to protect the Tailed Beasts, they were seen as tools to humanity and would be abused. Gyūki was hidden in a temple that would later be part of the Land of Lightning. Gyūki and the other Tailed Beasts were told before their separation that they were all linked despite being separate beings, and that one day, they would be reunited.
Long before Gyūki would become reunited with his siblings, Hashirama captured eight of the Tailed Beasts and sold Gyūki and Matatabi to Kumogakure, as part of a deal to balance the strength of the nations. Since then, Gyūki had been made to produce jinchuuriki of the Hidden Cloud Village, alongside Matatabi. Murals of Gyūki and Matatabi were made in a cavern behind the Falls of Truth, indicating their association with the Hidden Cloud Village. The nation struggled to find anyone who could control Gyūki, who would break free of its jinchuuriki and go on rampages, forcing the nation to seal Gyūki and unfortunately kill its jinchuuriki in the process. During one instance, the Third Raikage was pushed to his limits fighting Gyūki, and ended up shaming himself by receiving a self-inflicted wound while fighting him. The last victim of Gyūki was Blue B, the nephew of the Third Raikage.
Several of these attacks led to the formation of the Eight-Tails Subduing Team, which was formed by the Third Raikage, consisting of the Raikage and several jonin. In the process of trying to seal Gyūki after it escaped from Blue B, Furui was impaled by one of its horns, but posthumously succeeded in helping to stop Gyūki. The Raikage bravely selected B to be the next jinchuuriki, which caused a rift between him and Motoi, who became bitter about his father being killed by Gyūki. Motoi one day attempted to kill B, but relented after realizing that being the jinchuuriki of Gyūki already puts him in so much pain. Gyūki became feared in Kumogakure, which caused B to be shunned. Despite this, B befriended Gyūki after training rigorously within the Falls of Truth to become strong enough to control it, and the Tailed Beast no longer became a threat to Kumogakure.
Personality[]
Gyūki is quite affable after being tamed by Killer B, which is a far cry from its barbaric demeanor that terrorized Kumogakure. After having been sealed into several shinobi, Gyūki became hateful towards humans and took over all its jinchuuriki and went on rampages. It became infamous for its violence and destructive capabilities, and every one of its jinchuuriki suffered and died because of him. After being defeated and befriended by Killer B, Gyūki stopped being a misanthrope, and adopted a more relaxed persona, unlike Kurama. It is very good friends with Killer B, being unafraid to voice his honest opinions of B, but still remaining great friends with him. Gyūki acts as a voice of reason to B, being more perceptive and strategic. Towards his former host, Blue B, Gyūki acted just like Kurama, hoping to exploit his negative thoughts to possess him, but after being redeemed, stated that he no longer has any ill will towards Blue B. He became considerably kind after becoming friends with him, not just wanting to protect him, but other people as well. During his time with B on the Island Turtle, he agreed to teach Naruto how to control Kurama, saying that despite Kurama being arrogant and antagonistic towards other Tailed Beasts, it insisted that Naruto and Kurama should still be taught how to cooperate.