Killer B is the jinchūriki of Gyūki — the Eight-Tails. He is capable of transforming into various forms that resemble his tailed beast, until he finally takes on the complete form of Gyūki. So far, he has displayed a greater variety of forms than any other jinchūriki. Unlike most jinchūriki, he has full control of Gyūki's power. According to some Kumogakure shinobi who passed by and saw his full Eight-Tails form, the Fourth Raikage had forbidden him from fully transforming.
Using his own chakra, Killer B can freely and quickly form a red demon cloak that surrounds his body, which creates his "Version 1". Once activated, his canine teeth and nails extend. The cloak itself gives Killer B paws of chakra around his hands and horns on his head. He can also create an additional set of horns on his forearms to use for his Lariat attack. The attack power of this form is very high as Killer B can either decapitate a person with a single smash of his forehead or if his Lariat connects he can completely blow away their entire chest causing massive damage, and even destroy a massive platform where he trains.
He enters a three-tailed form during his fight with Taka; and one, four, seven, and eight-tailed forms at different points during his battle with Kisame. Kisame's Samehada is able to absorb up to six of these tails at a time. Like Naruto's first three tailed forms, these tails are translucent with bubbles of chakra forming on the surface. Other than an incremental increase in Killer B's strength and speed, there are no obvious differences between the number of tails.
By tapping into Gyūki's chakra reserves, Killer B can enter his "Version 2" (バージョン2, Bājon Tsū) forms. As stated by Sabu, this is done by forming the chakra into a human shape. These transformations are meant to give him an edge in battle without completely releasing Gyūki, as that would endanger anyone around him. To transform into Version 2, B forms eight Version 1 tails and wraps them around his body transforming him into Version 2, creating a crater underneath him. He can also enter Version 2 from his full Eight-Tails form.
The version 2 chakra cloak is much darker than version 1. In this form B's overall appearance changes drastically and while in this form, B becomes more muscular, and if necessary can create chakra bones, such as the bull's skull, to increase the strength of his attacks.
When struck by Samehada, only Gyūki's chakra is absorbed, so Killer B reverts to a version 1 form with the same number of tails. So far only the version 2 eight-tailed form has been seen, so it is unknown whether he can activate version 2 with fewer than eight tails. The amount of chakra in this form is so great that when Nagato absorbed it, it returned him to his appearance before he had used the Summoning: Demonic Statue of the Outer Path technique.
Killer B has an extremely high degree of control over Gyūki's power, to the point where he could not only create different versions of the tailed beast chakra cloaks, but limbs of the beast itself. When partially transforming his fist, he could send Might Guy flying over half the length of the Island Turtle with a single toss, as well as catch A's punch without receiving any damage. Partially transforming to use Gyūki's tentacles, he gains a high degree of dexterity, witnessed when he stacked geometrically shaped boulders one on top of the other and his free use of the tentacles like extra limbs.
Tailed Beast Mode
While fighting Taka, Killer B managed to wound Sasuke long enough to enter Tailed Beast Mode (尾獣モード, Bijū Mōdo), thus fully transforming into Gyūki, in which his skin is destroyed in a similar fashion as Naruto when he reaches the fourth tail and above. Although he was forbidden to fully transform by his brother due to its immense power, B tends to ignore this order, as he transforms to train or to fight formidable foes, above other reasons.
In this form, he is able to create a Tailed Beast Ball in front of his mouth and releases it as a widespread blast, causing destruction on a large scale. He can also create tremendous winds which can level a significant part of a forest with ease. In this form he is immensely durable, able to endure Amaterasu for a considerable amount of time- long enough to perform a Body Replacement Technique with a tentacle. He is also extremely fast in this form, able to block Amaterasu with his fist and strike extremely quickly with his tentacles. In addition, Killer B also has shown to have as much physical strength as Gyūki, enough to catch Kurama's Tailed Beast Ball and block the full blast from hitting Naruto.
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