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 and it killed eight Kumogakure 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. 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.

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. He was also presumably skilled in kenjutsu as he carried a sword around with him. In the anime, his talents as a shinobi and Jinchūriki were great enough that Kabuto chose to reincarnate him for the Fourth Great Shinobi war.

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."