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. After A's cousin, the jinchuuriki of Gyūki lost control and killed Motoi's father along with seven other ninja on that day, B was chosen as its next Jinchuuriki at the age of 5.
During the 3rd Great Shinobi World War, he and his brother encounter Minato Namikaze and had a small scuffle with Minato and his squad.
Killer Bee is a muscle-bound shinobi from Kumogakure and the Jinchuuriki of the Eight-Tailed Beast. It is hinted he comes from the Yotsuki clan. He appears to dislike light. On his right shoulder, he has a tattoo that says "iron" (鉄, tetsu); and on his left cheek, he has a tattoo of a bull's horn (correspondingly, the Eight-Tails has horns like a bull, and the left one is missing). He also carries seven swords and has a brother who is the Raikage.
It seems that he demands respect from others and asks to be called "Lord Jinchuuriki" (人柱力様, Jinchuuriki-sama) and his Tailed Beast be called "Lord Eight-Tails" (八尾様, Hachibi-sama). He also appears willing to speak in song, more specifically in rap style, even when confronted with potential threats. He will even stop in a middle of a fight to write whatever rhymes come to him. He has a casual relationship with the Eight-tailed Beast, frequently engaging in friendly conversation with it.
As a Jinchuuriki, Killer B has a massive chakra pool enough to make Samehada get very excited and betray Kisame, who is known for his massive amounts of chakra. Killer B is one of the few people to have achieved full control over a tailed beast and is known as a Perfect Jinchūriki because of this feat along with Yagura, the Fourth Mizukage and Naruto Uzumaki.
B is the only known, as well as living Jinchuuriki who can at will, transform and remain in his full tailed beast form indefinitely. Because he has a strong relationship with his own tailed beast, it acts as his partner in battle, disturbing his chakra to dispel even genjutsu from the Mangekyou Sharingan.
(Source: Naruto Wikia)
