Blue B

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

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 Killer B 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.

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 ability. As the jinchūriki of the Eight-Tailed Giant Ox, he was able to use it 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; which he ultimately failed in.