Blue B

was a high-ranking Kumogakure shinobi as well as the predecessor of Killer B as the jinchūriki of Gyūki, the Eight-Tails.

Background
Though hailed as an excellent shinobi, Fukai didn't have complete control over the Eight-Tails, a fact he lamented while on a mission with A, Killer B and another unknown shinobi. He stated that neither him nor his uncle, father, and predecessors could subjugate out of 'incompatibility'. He however hoped that B, should he be chosen as the next host, would be able to successfully control the beast. As A wondered if B would have enough control to be able to suppress the beast, he told him of the darkness and loneliness that was part of being a jinchūriki as even people that you knew all your life would grow distant. A protested calling him a man of great talent, virtue and lineage, he told him that none of that mattered and what a jinchūriki truly needed was something to give them strength and fill the void that would be created after the beast was sealed into them.

Some time after this, he lost complete control of the beast and it killed eight Kumo ninja, including Motoi's father who was a member of the subduing team. The Third Raikage extracted the Eight-Tails sealing it into the Kohaku no Jōhei, resulting in the jinchūriki's death. In the anime, it was revealed that Orochimaru caused the events leading to Fukai's death by posing as his doctor and giving him genjutsu inducing pills to draw out the tailed beast and retrieved the severed horn of the Eight-Tails with Fukai's DNA in it. The villagers later lamented on Fukai being a failed jinchūriki at his funeral while B paid his respects.

Personality
Fukai strongly believed that once a person became a jinchūriki, their life would be full of darkness, fear and overwhelming loneliness. He also believed that what a jinchūriki truly needed was to have something that filled the void in their heart and give them strength. The prejudice that he was shown after having the beast sealed into him led him to lose his will to live at first due to the overwhelming fear and loneliness he felt. Fukai was even willing to accept the fact that he himself was incapable of controlling the tailed beast within himself.

Appearance
Fukai had long hair that his half green and half white hair, dark, heavy-lidded eyes that gives him the appearance of being very tired and a purple mark in the shape of a diamond on the right side of his face. He wore a Kumogakure flak jacket over a long sleeved V-neck outfit with bandages underneath and carried a fairly long sword strapped to his back. In the anime, while entering "Version 2" form, the blue diamond shape on Fukai's eye can be seen and the left horn is coloured green.

Abilities
Although nothing has been seen of his abilities in the manga, A hailed him as a man with an aptitude for ninjutsu and one who possessed great abilities. . In the anime, his talents as a shinobi and jinchūriki were great enough that Kabuto Yakushi chose to reincarnate him for the Fourth Shinobi World War.

He was also presumably skilled in kenjutsu as he carried a sword around with him.

Taijutsu
In the anime, Fukai demonstrated considerable unarmed and close-ranged fighting prowess, able to easily fend off multiple Iwa ninja. After being reincarnated, he was able to fight Killer B to a standstill while they were both in their Version 2-state. He could also perform the Lariat, with enough power to match Killer B's own.

Jinchūriki Transformations
As a former jinchūriki, Fukai was blessed in like with immense reserves of equally powerful chakra. While never obtaining complete control of Gyūki's power, Fukai was known for his impressive use of it. Fukai was shown able to at will access his chakra shroud, entering into "Version 2" form with eight tails and still retain control of his actions. He could also manifest a bone structure to aid him in the Lariat and attack his enemies from a distance by extending his chakra cloaked arms. His might in this form was able to fight on par with Killer B, the current jinchūriki. He could perform the Tailed Beast Ball and rapidly fire them. In this form, he could easily resist strong electrical blasts. He could also communicate mentally and talk with Gyūki and B.

In the anime, after being reincarnated, while having lost access to Gyūki specifically, Kabuto used the harvested genetic material from Orochimaru's experiments with to restore most of Fukai's previous jinchūriki powers.

Shinobi Wold War Arc
In the anime, seeing an opportunity to capture Killer B, Kabuto summoned Fukai to the battlefield. His personality restored, Fukai was surprised to see how much B and Motoi had grown. He was told about his final rampage as a jinchūriki, sincerely apologising for killing Motoi's father. Unable to control his actions, Fukai surprises his former allies with his intact ability to use Gyūki's chakra. Upon entering "Version 2", Fukai quickly showed off his prowess as a jinchūriki, forcing B to enter "Version 2" as well. When Gyūki insisted on how he could still have access to its power despite no longer residing in him, Fukai revealed that the Eight Tails's severed horn was a medium that restored his jinchūriki powers while revealing that Orochimaru is the one who masterminded the rampage thirty years ago. While B and he were equal in power, Fukai's reincarnated body gave him the edge with B forced to retreat. Motoi attempted to hold off Fukai, only to be quickly defeated. Seeing his friend in trouble, B ceased his retreat and counter-attacked. Surprised at how passionately B wished to help Motoi, he realised that B had found friends very special to him. After B told him that he also is one of those dear to him, Fukai was in peace that his successor found something in life as a jinchūriki and is able to break the summoning contract and return to the afterlife.

Trivia

 * "Fukai" can mean, , and.
 * Fukai's name originates from the anime. To date, he has not been given a name in the manga.

Quotes

 * (To A) "Something to fill up that hole in their heart… Anything!!! If they can just find something… something to give them strength… help B find that something."