Blue B

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

Background
Though hailed as an excellent shinobi, he didn't have complete control over the Eight-Tails, a fact he lamented. His uncle and his father before him were also incompatible with the beast and he hoped that his cousin B would be able to successfully control the beast. While on a mission, he told A that if Killer B was chosen as the next jinchūriki, he would need something to fill the void that would be created after becoming a jinchūriki.

At some point he lost complete control of the beast and it killed eight Kumogakure ninja, including Motoi's father. The Third Raikage extracted the Eight-Tails, resulting in the jinchūriki's death. The Eight-Tails was later sealed in Killer B. Also according to A, he was a man of great virtue as well as lineage.

Personality
He strongly believed that once a person became a jinchūriki, their life would be full of darkness, fear, and loneliness, and what a jinchūriki truly needed was to have something that fills the void in their heart and give them strength. He was even willing to accept the fact that he himself was incapable of controlling the tailed beast within himself.

Appearance
He had long half-black and half-white hair with matching eye brows, dark eyes and a tattoo 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.

Abilities
Although nothing has been seen of his abilities, A hailed him as a man with an aptitude for ninjutsu and possessed great abilities. As the jinchūriki of the Eight-Tails, he was able to use its powers, but he could only manage as much as two tails before losing control.

Trivia

 * This person's status as a jinchūriki suggests he might have had relationships to former bearers of the title of "Raikage" (as per tradition) or simply a competent enough shinobi expected to use the tailed beast's power effectively.