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 father 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 later, in the anime, he was sent on a mission with A and B where they confronted several Iwa ninja, after defeating them Fukai explained to an exhausted B where A's strength came from.

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 subdued the Eight-Tails by sealing it into the Kohaku no Jōhei, resulting in the jinchūriki's death. In the anime, it was Orochimaru who caused the events leading to Fukai's death by posing as his doctor and giving him genjutsu inducing pills to paralyse him and proceeded to use the Five Elements Unseal to draw out the tailed beast. Later, during the battle that followed, Orochimaru retrieved traces of the Eight-Tails' severed horn along with Fukai's DNA in it. The villagers later lamented 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.

He had known B and Motoi since they were young, and while finding B's rhyming sub-par he firmly believed B was the best suited to become Gyūki's next jinchūriki. In the anime, he was shown to be visibly saddened to learn that eight of his fellow Kumo ninja had died trying to stop his rampage, and apologised to Motoi after being informed his father had been among them, showing that he took responsibility for his actions. Fukai was glad to realise B had found the "something" a jinchūriki needs to be happy, and congratulated his successor for succeeding where he had failed.

Appearance
Fukai had long hair that is half green and half white, dark, heavy-lidded eyes that give him the appearance of being very tired and a purple mark in the shape of a diamond on the right side of his face set around his right eye. 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. After being reincarnated, he looked like he did while alive with the exception of pupil-less eyes and grey sclerae.

Abilities
Although nothing has been seen of his abilities in the manga, A hailed him as a highly skilled shinobi with a natural aptitude for ninjutsu. 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. In the anime, he was also shown to be a sensor-type ninja, able to detect Gyūki's chakra within B despite his successor not accessing the tailed beast's chakra at the time.

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 and use the Lariat with enough power to match B's own while they were both in their Version 2 state.

Jinchūriki Transformations
As a former jinchūriki, Fukai possessed immense reserves of powerful chakra. While never obtaining complete control of Gyūki's power, Fukai was known for his impressive use of it. When he was alive he was able to use partial transformation by manifesting one of Gyūki's tails. In the anime after being reincarnated, while having lost access to Gyūki specifically, Kabuto used Gyūki's genetic material, which was harvested through Orochimaru's experiments, to restore most of Fukai's previous jinchūriki powers, becoming in essence a pseudo-jinchūriki.

Fukai was shown to be able to access his chakra shroud at will, entering an eight-tailed Version 2 form while still retaining his personality. While in this form, Fukai becomes noticeably more muscular, the blue diamond shape on Fukai's eye can be seen, and the left horn is coloured green. He could also manifest a bone structure to aid him in performing the Lariat and attack his enemies from a distance by extending chakra arms. His might in this form was enough to fight on par with Killer B, Gyūki's current jinchūriki, as well as easily resist strong electrical blasts. Fukai could also perform the Tailed Beast Ball as well as rapidly fire them without fully transforming into the Eight-Tails. He could also communicate telepathically with other jinchūriki.

Shinobi World War Arc
In the anime, seeing an opportunity to capture Killer B, Kabuto summoned Fukai to the battlefield. After being summoned, 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 knowing how he could still have access to its power despite no longer residing in him, Fukai revealed that the Eight Tails' severed horn was a medium that restored his jinchūriki powers while also revealing that Orochimaru is the one who masterminded the rampage thirty years ago. While B and he were equal in raw 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, to which B told him that he also is one of those dear to him. Finding peace that his successor found that missing "something" in life as a jinchūriki, Fukai 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."
 * (To B) "You found them. Lots of special friends, that "something" to fill the hole in your heart."