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 or his father before them were able to subjugate the beast and stated that they were 'incompatible'. 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 (in the anime it was due to Orochimaru's infiltration of Kumogakure) 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 Orochimaru was responsible for the Eight-Tails rampage, having given Fukai genjutsu inducing pills to draw out the tailed beast. After the Eight-Tails attack, Orochimaru received some of Fukai's DNA from the cut Eight-Tails horn to further his experiments. The Eight-Tails was later sealed in Killer B some years after this. At the funeral held for him, the villagers lamented that he too was a failure in the end, and as B paid his respects to the deceased jinchūriki, the villagers deemed that the young man would also be a failure.

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 dull-black 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, 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.

Jinchūriki Transformation
As a former jinchūriki, Fukai was blessed in like with immense reserves of equally powerful chakra. While never obtaining complete control of Gyūki, Fukai was known for his impressive use of its power. Fukai was shown able to at will access his demon cloak, 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 killed Motoi's father. Unable to control his actions, Fukai surprises his former allies with his ability to still access the Eight-Tails demon cloak. Entering "Version 2", Fukai quickly shows off his prowess as a jinchūriki, forcing B to enter "Version 2" as well. Gyūki insists to know how Fukai can still have access to his power as they are now no longer together. Fukai explains that the severed horn of Gyūki was used as a medium to restore Fukai's former power. He also explains how it was Orochimaru who caused Fukai to go on a rampage thirty years ago. While B and Fukai are equal in power, Fukai's reincarnated body gives him an edge that forces B to retreat. Motoi attempts to hold off Fukai, only to be quickly defeated. Seeing his friend in trouble, B ceases his retreat and counterattacks. Surprised at how passionately B wishes to help Motoi, he realises that B has found friends very special to him. As B tells Fukai that the late shinobi was also one of the special people to him, Fukai is glad to learn that B has found in life as a jinchūriki what the predecessor could not. Finding peace in himself, Fukai was able to break the summoning contract and return to the after life.

Trivia

 * "Fukai" can mean, , and.
 * Fukai's name originates from the anime. To date, he has not been given a name in the manga and remains the only unnamed character in the manga to be given one in the anime exclusively.

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."