Killer B is the guardian of Kumogakure, the jinchuriki of Gyuki, the Eight-Tails, the younger adoptive son and brother of the previous and present Raikage.
Killer B was born into Kumogakure twenty-one years before the start of the series. When he was five, B became the younger adopted brother of A, the future Fourth Raikage at the time, when he managed to successfully perform the Double Lariat with A, and was given the name "B". After B's cousin, the jinchuriki of Gyuki, the Eight-tails, at the time lost control and killed the father of Motoi, B's best friend, B was chosen to be the next jinchuriki of the beast when his adoptive father sealed it. After B became the new jinchuriki, he began to suffer unjust treatment from the people of Kumogakure, but remained happy and determined to be the best jinchuriki he could be for the sake of his brother. Motoi once tried to kill B in order to avenge his father by killing Gyuki, but B easily disarmed him and held him no ill will, despite the attempt on his life, causing Motoi to not speak to B for thirty years out of shame. B went on to train with his brother on the Island Turtle, where he learned how to control Gyuki in the blink of an eye at the place called the Falls of Truth. In later years, B and his brother, A and their team encountered Minato Namikaze, who later became the Fourth Hokage of Konohagakure, and B blocked him with one of Gyuki's tentacles when he attempted to counter A's attack, shocking the Konoha ninja present and ultimately they fled when Minato and B's confrontation reached the point of a stalemate. B has since become a beloved jinchuriki in Kumogakure, much like Naruto has in Konohagakure, with Motoi calling him the "Hero of Heroes".
Killer B first appeared in the Valley of Clouds and Lightning after a half a day of training and as he left for lunch, he was confronted by Taka, led by Sasuke Uchiha, who were working with the Akatsuki, a criminal organization, and looking to take him captive for the sake of power. B was at first attacked by Suigetsu Hozuki, whom B stopped and disarmed of his sword, Kubikiribocho, which he then used to overpower Jugo when the latter attacked him. Throwing Jugo back to his group, B is confronted by Sasuke himself, who took back Suigetsu's sword. Impressed, Killer B drew all seven of his swords and proceeded to attack Sasuke with his unique Seven Swords Dance, which Sasuke's Sharingan failed to read through, allowing B to slash him four times and repel him. When Sasuke then tried to funnel lightning-natured chakra through his sword, B did the same with one of his, blocked the attack, and stabbed his other six swords into Sasuke's torso, critically wounding him. Before B could finish Sasuke, his teammates came to his rescue and drove B back, allowing Karin to heal Sasuke and resume the battle. B then took on Sasuke, Jugo, and Suigetsu together and managed to evade them, despite being struck by a water-conducted Chidori. Bored of the battle, B tried to leave, but was tracked by Karin's Mind's Eye of the Kagura. Escape not an option. B got serious about the battle as Sasuke charged him.
B drew on some of the chakra of Gyuki, his tailed beast, and created a chakra cloak before attacking Taka, but they managed to evade him. As B charged Sasuke, the latter caught him in Tsukuyomi and knocked him out. However, Gyuki broke him out of it and B retaliated on the unaware Sasuke with great force, using his enhanced Lariat move to blow Sasuke's chest open, again critically wounding him. B then proceeded to fully transform into Gyuki after pointing out that genjutsu is useless against him due to his partnership with Gyuki. As B charged the horrified Taka members, he is blocked by Suigetsu, who protects the team from B's Tailed Beast Ball, being knocked out in the instance. Sasuke, then specially healed by Jugu, used Amaterasu on B, causing him to rampage in pain and nearly crush Karin with one of his tentacles, which was severed by Sasuke. B used the moment to substitute with the tentacle and fool Taka with a decoy to allow him to leave Kumogakure with Gyuki for some vacation time, allowing his brother, the Raikage, and the rest of the village to believe he was kidnapped.
B sought out Sabu, an Enka music singer, who was to be his teacher in the art of the music, but he is found by Kisame Hoshigaki of the Akatsuki, who was there to do what Sasuke couldn't. B, unable to use his full strength due to the presence of Sabu and his pet, Ponta, was unable to fight as an equal to Kisame, whose Samehada absorbed B and Gyuki's chakra all the while. B managed to critically wound Kisame after using version 2 of his chakra cloak, but Samehada absorbed it back to version 1 and healed Kisame, who trapped B, Sabu, and Ponta inside a giant water dome before merging with Samehada and proceeding to attack the group. With Gyuki's help, B managed to free Sabu and Ponta by restraining Kisame with tentacles and pulling him away, forcing the water dome to move with them as Gyuki stated it would. Kisame, however, still absorbed B's chakra and beat him unconscious. When he tried to cut off B's legs after releasing the water dome, Samehada betrayed Kisame an gave B some chakra. B then threw a lightning-enhanced pencil and pierced his cheek when Kisame tried to use B's sword against him and A, B's brother, the Raikage, and his bodyguards arrived and stopped Kisame, allowing Samehada to restore B's chakra reserves. B and A then used their Double Lariat move to decapitate Kisame(really just a clone made by Zetsu, another Akatsuki member. The real Kisame was hiding within Samehada.). B then took Samehada for his own and returned home with his brother and fellow Kumo-nin.
Later, B is sent to the Island Turtle during the outbreak of the Fourth Shinobi World War, along with Naruto Uzumaki and several Konoha and Kumo ninja. B is approached by Naruto, who wanted to learn to control Kurama, the Nine-Tails, but B refused to train him at first due to lack of readiness on Naruto's part. After B reunited with Motoi, B's best friend from childhood, and reconsiled with him, B took Naruto, then prepared to train to control Kurama, and his teammate, Yamato, to the Tailed Beast Temple behind the Falls of Truth and joined the two in the training room. B aided Naruto in his battle with Kurama until he was knocked out of Naruto sunconscious, but instructed Naruto all the while and was impressed when Naruto defeated Kurama and attained the Nine-Tails Chakra Mode. After Kisame Hoshigaki was discovered and defeated by Might Guy before commiting suicide, B began to train Naruto in how to fully transform into Kurama and, when that proved impossible at the time, in how to use the Nine-Tails Chakra Mode in order to keep him occupied as B was aware of the war and Naruto wasn't. After Naruto finds out about the war going on and tries to leave, B has a change of heart about the war, aids him in his escape, and the two leave to join the war. Along the way, they are confronted by A and Tsunade, whose intensions to stop them from going any further. B tries to convince his brother to let them pass and counters him when he attempts to kill Naruto. When A doesn't listen to B continuously, B overpowered him when they struck each other with their Lariats and threw him to ground before telling his brother that his and Naruto's strength came from the people that believed in them, and not just from their tailed beasts, prompting A to test Naruto after pushing B aside. After Naruto dodges A, impressing B, A allows them to pass and join the war.
B and Naruto later encounter Itachi Uchiha and Nagato, both revived by Kabuto Yakushi, after they both had done some activity in the war. A sudden battle ensued as Naruto began to converse with the two and B bisects Itachi's Fire Release: Great Fireball Technique and counters his attack from behind with one of Gyuki's arms before using a tentacle to grab Itachi, but it was just a genjutsu. As Itachi warned B not to look at his eyes as he was about to use Tsukuyomi against his will, Gyuki broke B out of the genjutsu and cancelled out the oncoming Tsukuyomi, allowing B to draw all five of his swords he had stored inside Samehada and form his Seven Swords Dance to attack Itachi, forcing the latter to flee to Nagato's summoned bird. B threw a sword as Itachi activated his Mangekyo Sharingan, but it was repelled by Nagato. At that moment, a crow emerges from within Naruto and casts Kotoamatsukami on Itachi, allowing him to break free from Kabuto's control and join B and Naruto's side against Nagato, surprising B. B, Itachi, and Naruto work together against Nagato, who had regained his mobility and youthful appearance after absorbing some of B's Eight-Tails chakra when overpowering both him and Naruto with his Rinnegan abilities. They are rescued by Itachi and the trio work together to destroy Nagato's Planetary Devastation, allowing Itachi to seal him with the Sword of Totsuka. B promises Itachi to look after Naruto before the latter left to stop Impure World Reincarnation. B later stops to urinate and lets Naruto go ahead.
B later catches up with Naruto shortly before they encounter Tobi and his Six Paths of Pain made out of the revived jinchuriki who once again hosted their tailed beasts. As they begin to battle the revived jinchuriki, B converses with Gyuki about their fates, especially that of fellow Kumo shinobi Yugito Nii. B was later surprised to see they still had their tailed beast abilities then and is surprised further when some manage to dodge his attacks repeatitively. After Naruto points out they share vision with the Rinnegan, B caught Naruto when he was hit and fled the vicinity from Yugito's Mouse Hairball technique, damaging her as he escapes, allowing B to discover Tobi's chakra receiver rod on her and the other jinchuriki. After Naruto fails to destroy the chakra receiver in Roshi, Son Goku, the Four-Tails' jinchuriki's chest, B fully transforms into Gyuki and levels the forest with a whirlwind to clear the area of trees. B then attempted to seal the revived jinchuriki with his Octopus Hold technique, but fails when Tobi makes them enter Version 2-like states. B and Naruto then began to fight the jinchuriki, but quickly begin to feel the effects of fighting six very powerful opponents at once. B is gouged by a fully-transformed Han and repelled, but regains consciousness and asks Gyuki if it was alright before attempting to crush Tobi after he restrained Kokuo, the Five Tails, when he lost control over it and restrained it with a chakra chain, but Tobi slips through his fist. Kakashi Hatake and Might Guy soon arrive as back up against Tobi.
B later urges Naruto not to use shadow clones when he suggests them to help strike Tobi as it would kill him with Kurama sucking away his chakra, but Gyuki point's out that it had stopped or Naruto would already be dead. B is then tossed away by the fully-transformed Roshi and was impressed when Naruto managed to escape from Son Goku's belly using shadow cloning and was shocked when Tobi sucked it into the Demonic Statue of the Outer Paths after Naruto frees it. B is then astonished when Naruto fully transforms into Kurama after becoming its partner and the two confront the revived jinchuriki, all fully-tranformed at that point into their tailed beasts. As the beasts formed a collab Tailed Beast Ball, B grabs Roshi's body, then disabled after Naruto freed Son Goku, and protects it and himself as Naruto collides a Tailed Beast Ball of his own with theirs and manages to dispel it. After Naruto frees the other tailed beasts, B congradulates him and joins Naruto, Guy, and Kakashi to fight Tobi directly after he reseals the tailed beasts into the Outer Paths statue. After B sees the corpses of the revived jinchuriki start to deconstruct, he realizes the Impure World Reincarnation had been released and tells Naruto they will have to pick up the pace to compete with Itachi's contribution to the war shortly before Tobi revives an incomplete version of the Ten-Tails. After Tobi's mask is gashed, the group discuss how to defeat him and B attacks the Outer Path statue, but his punch was deflected by Tobi's Uchiha Flame Battle Encampment technique. B then threw Naruto, Guy, and Kakashi at Tobi and later listens to Kakashi explain Tobi's intangibility technique after Tobi's right arm is wounded by a seemingly unseen attack. After B questions how Kakashi and Tobi's Kamui and Space-Time Migration could share the same demension, the battle resumes and Tobi fires stakes at Gyuki and B that allow him to supress a tailed beast's power by using Space-Time Ninjutsu, but B manages to break free from their hold, with aid from Naruto, after Madara Uchiha arrives on their battlefield and joins Tobi.
Engaging Madara in battle, B is defeated by Madara, alongside Might Guy, causing B to praise his strength. Breaking free from his Wood Release restraints, B aides Naruto in the creation of a gigantic Tailed Beast Ball, which they shoot at the incomplete Ten-Tails in attempt to destroy it before it revives, but fail as the beast revives regardless. Teaming up with Naruto to attack the Ten-Tails, B and Naruto are both repelled by its attack. Afterwards, B and Naruto both fire off a barrages of Tailed Beast Balls at the Ten-tails, and narrowly dodge its own, which effortlessly overpowers their efforts. Teaming up with Kakashi, B allows him to such him up into his Kamui demension and when Kakashi brings him out above the Ten-Tails, B prepares to fire a Tailed Beast Ball at its eye from point-blank range. However, as he fires, the Ten-Tails thumps it back and it hits and explodes on him, dealing him multiple injuries. At his limit, B reverts back to his human form to rest. After he recovers, he leaps off of Sai's art bird and jumps at the matured Ten-Tails, transforming as he does and ramming his head and its Tailed Beast Ball in its mouth and firing both down its throat, repelling them apart in the resulting explosion. After Madara launches another attack with the Ten-Tails chakra arms, Killer B uses his Tailed Beast Bomb together with several other alliance members and overpowers his attack. When Gyuki tells him its time to get busy when Naruto is knocked out of the fight, B agrees, listening to Gyuki's warning that they must stop the Ten-Tails before it completes its final transformation. After Shikamaru asks him to alter the Ten-Tails gigantic Tailed Beast Ball as high as possible, B transforms into Gyuki and tries to hold the Tailed Beast Ball back with his tentacles after it is fired. However, Minato Namikaze, Naruto father and the Fourth Hokage, arrives and uses his Flying Thunder God Technique to teleport the Tailed Beast Ball away. Later, after Obito becomes the Ten-Tails jinchuriki, B asks Gyuki if that was the beast's final form, to which it answers that it was not, just the one before it.
After Obito manages to destroy the Four Red Suns Battle Encampment technique as well as Hashirama Senju's restraints, B asks Gyuki if it seems like the Ten-Tails had gotten strongest in this stage, to which he answers that it had not been using its power seriously until this point of the battle. After Naruto's earlier given chakra cloak reappears with a new, dark tent to it, B converses with Gyuki, who states it was not Kurama. After Obito releases the Ten-Tails in its final form, the Shinju, and it begins attacking everyone and stealing their chakra, B, lamenting on the fact he had many more branches from the Shinju coming for him, is informed of what it was by Gyuki and that the reason he has so many is because it probably can sense he has a lot mmore chakra than some of the others. After destroying the branches after him with Samehada, B is left tired afterwards. After his brother, A, arrives, B declares it had been a while and that it was time to fight with their Double Lariat, mobilizing with him and the other Kumogakure shinobi. Later, however, when Naruto and Sasuke attempt to pull the tailed beasts out of Obito, being that Naruto lacked any of Shukaku or Gyuki's chakra, B and Gaara joined in the pull, using one of his tentacles to pull out Gyuki's chakra and Shukaku from inside Obito, with most of the alliance joining in as well, and them succeeding in removing the beasts. After this, B fully transforms in Gyuki and takes the injured to safety, before joining Naruto, Gaara, and the tailed beasts in fighting the now resurrected Madara Uchiha, them all using a tail to smash his Susanoo and knock off his right arm. This doesn't kill Madara, however, and when he quickly regains his right Rinnegan, B, Naruto, Gaara, and the beasts are repelled by his Rinbo: Hengoku technique and bound around their necks by the Demonic Statue of the Outer Path's chains. To B and Naruto's terror, Madara then announces he will remove their tailed beasts from within them, with one of B's tentacles being later cut off by Madara's Susanoo sword as he and the other beasts were dragged towards the statue, with Madara sealing the all of them, including Gyuki inside. Despiting appearingly losing his Tailed Beast, B manages to survive by making use of the chakra leg of Gyuki's that was hacked off earlier, with him soon being shocked when Madara activated his Infinite Tsukuyomi, which he likely fell prey to.
Killer B has a typical rapper personality in that he frequently talks in rhyme and is normally very goofy and carefree, but is capable of being very serious when its needed. Like Naruto, B can be quite cocky and stubborn and demands respect from others, demanding to be called Lord Jinchuriki and that his tailed beast be called Lord Eight-Tails. B has a great deal of pride in his power and of who he is, but is still able to recognize talent when he sees it and respects those with it. B has a strong relationship with his tailed beast and they are partners in battle, with Gyuki providing various forms of aid when B needs it. B is also very close to his older brother, A, but is not above disobeying him as, despite A's order not to fully transform into Gyuki, B regularly does so when he can such as when training or to battle more powerful enemies.
As the jinchuriki of Gyuki, the Eight-Tails, and the guardian of Kumogakure, Killer B is an extraordinarily powerful ninja, capable of overwhelming and defeating all of Taka by himself easily and survived his battle with them without getting a scratch on him. A, the Fourth Raikage, has praised B's skills and claimed B had more talent that even him. Tobi has also praised Killer B as the perfect jinchuriki for his perfect mastery of tailed beast control.
Killer B is the most skilled swordsman in the series, able to weild seven swords at once in a unique fighting style called the Seven Swords Dance. Because of the unpredictable form of this swordplay, most of his opponents are easily overpowered by B. Neither Sasuke Uchiha, a Sharingan user and expert swordsman, or Suigetsu Hozuki, a highly trained swordsman by the Seven Ninja Swordmen of the Mist, could read B's swordplay, with the former nearly dying to B. Even Itachi Uchiha, a Konoha Anbu, all of whom are expert swordsmen, and an even greater Sharingan master than his brother, Sasuke, was not able to fight against B when he used this sword style, and was reduced to only dodging capability and forced to retreat. After his battle with Kisame Hoshigaki, B took possession of Samehada, which B can easily work well with and weild very proficiently. He carries it on his back, along with two other swords and carries the other five within it.
In terms of nature transformation, B is also very skilled in Lightning Release, able to funnel it through his sword and increase its cutting power, just like Sasuke Uchiha can his. He can also funnel it into a pencil and make it more powerful than a Wind Release chakra enhancement, able to pierce through the trunk of a tree easily. In terms of physical prowess, B has immense physical strength, able to catch the blade of the Kubikiribocho and disarm Suigetsu Hozuki of it instantly, weild it effortlessly, despite not having been trained to, which baffled Suigetsu, and even overpower his brother's Lariat while the latter was clad in his Lightning Release Armor using only his own raw strength Lariat. B has incredibly fast reflexes, able to instantly react at a moments notice to Sasuke Uchiha attacking him at close range and block and repel him with a single move, as well as instantly dodge Jugo attacking him from behind, despite having just been blasted by a water-conducted Chidori. His durability is so great, he take a direct punch in the face from Jugo while the latter was partially transformed into the second stage of his Cursed Seal without even getting a scratch on him, and even take a direct Lariat to the chest from his brother with no visible damage. B is also incredibly fast, able to make it to another mountain after dodging Jugo before Taka even saw he was gone, requiring Karin to locate him with Mind's Eye of Kagura.
Killer B has full control of Gyuki, the Eight-Tails,' powers and can freely draw on its chakra to suppliment his taijutsu abilities and even fully transform into it effortless in just a few seconds. While fully transformed, Killer B can access Gyuki's high-level chakra without losing himself even a little and use the Tailed Beast Ball, a jinchuriki's most powerful move, easily and was able demolish several mountains and create a giant lake in one attack while using it in his battle against Taka. B can also spin in a circle to create a whirlwind capable of destroying a forest almost instantly while fully transformed. Killer B's skill with partial transformations into Gyuki is such that he can grow all eight of its tentacles out of his back with or without a chakra cloak and can also grow one of its arms to throw someone a long distance or block very powerful attacks instantly. Due to the fact that Gyuki willingly helps B, his so called "control" over it is more of a willing partnership than actual control and due to this partnership, Killer B is invincible when it comes to genjutsu. Even those from a Mangekyo Sharingan user will fail against him, as both Sasuke and Itachi Uchiha have failed to successfully defeat B using even Tsukuyomi.