At a young age, B along with several other children were gathered in order to select a tag partner for A. Out of all the children gathered, B was the only person able to successfully perform the Double Lariat with A. As such he was given the name "B", and A stated they would be brothers from that day on. Since the age of five, B has been best friends with Motoi, whom he always greeted with a fist bump. After his cousin, the jinchūriki of Gyūki lost control and killed Motoi's father along with seven other ninja on that day, B was chosen as its next jinchūriki. Soon afterwards, B suffered from similar prejudices against jinchūriki, not too different from that the villagers of Konohagakure and Sunagakure had against Naruto Uzumaki and Gaara respectively. However, he kept smiling and pushed himself to be the best jinchūriki that he could be, so that his brother wouldn't be hurt politically as the Fourth Raikage.
After his father's death, Motoi tried to kill B, believing it would also kill Gyūki and avenge his father, but B disarmed him easily, and despite the attempt on his life, bore him no ill will, which led Motoi to not speak to B for thirty years out of shame. B would later train with his brother on an island in the Land of Lightning, where there he would find the Falls of Truth. There he learned how to control Gyūki in the blink of an eye. Some time after his training, B, alongside A, and other Kumo shinobi confronted Minato Namikaze and his small squad. When Minato moved to counter A's attack, B used a tentacle to push A out of the way, causing shock to the Konoha shinobi that he was Gyūki's jinchūriki. Some time since then, much like Naruto, B has become beloved in Kumo as the village's hero, or, as Motoi called him, the "Hero of Heroes".
When A became the Raikage, he forbade B from fully transforming into the Eight-Tails and restricted him from leaving the village, saying instead that he would become the guardian of the village and attack from within its walls.