Jūgo

Jugo is a shinobi cursed with mutative abilities that render him mentally unstable.

Background
Jugo from an unknown clan who have have inherited a means to absorb energy from nature like those who use Sage Jutsu. However, the ability to undergo various drastic physical alterations also results in sudden and uncontrollable surges of power that made them into berserkers. Jugo is no different as his rage made him a threat to anyone around him with little chance of survival should he lose his temper. Hating himself, Jugo took to secluding himself in caves and far from any human contact.

However, that changed the day Jugo met Kimimaro, who convinced him to come with him in hopes that Orochimaru would cure him. However, amazed by Jugo's abilities, Orochimaru instead tried to research Jugo's bloodline and learned of Ryuchi Cave where he obtained Sage Jutsu. But as Orochimaru can not use a perfect Sage Mode, he uses his Sage Chakra with an enzyme he extracted from Jugo's blood to create the Curse Marks. When Kimimaro died, meeting him prior to tell him that retrieving Sasuke Uchiha for their master would be his legacy, Jugo shut himself away once more within the confines of the Northern Base in the Hidden Sound Village.

Plot
Jugo was sought by Sasuke in the formation of Hebi to hunt down his brother Itachi Uchiha. He attacked at first, and refused to join Sasuke after learning of Kimimaro's death. He calmed down after learning Sasuke's identity and resolved to travel with him. Jugo saw Kimimaro's image in Sasuke and hoped that he would help keep him from entering his berserk state. After Itachi's death, and Sasuke learning the truth of his brother's past, Jugo and the other members of Hebi were reformed into Taka with a mission to capture Killer Bee, the Eight-Tails Jinchuriki. During the fight, Jugo regressed in age when he transfered some of his flesh into Sasuke to heal him.

Later sent along with his team mates to the Land of Iron, Jugo battles the Fourth Raikage, A, and his bodyguards. Knocked out, Jugo absorbed the body of a nearby samurai to regain his normal appearance before and and Suigetsu are captured and imprisoned by the samurai. As the Fourth Great Ninja War unfolds, Jugo and Suigetsu Hozuki managed to escape their captors and eventually reach one of Orochimaru's hideouts where Jugo accidently uncovers a secret room where he and Suigetsu find a scroll. Taking it to Sasuke, Jugo inspects the conscious Anko before helping Sesuke in reviving Orochimaru.

Later, when Sasuke was being tormented by thoughts concerning what he was, Jūgo and Suigetsu break through the ceiling and enter. Suigetsu examines Kabuto's body while Jūgo examines Anko, the latter stating that she is alive. When Sasuke asks them their purpose of coming there, Suigetsu hands him the scroll — the one with which he exclaims that their group Taka can rule the world with — and making him cognisant of reincarnated Madara's presence even though Itachi forced Kabuto to release Summoning: Impure World Reincarnation, which leaves Sasuke somewhat surprised. Jūgo later helped Sasuke in reviving Orochimaru by giving Sasuke a portion of Kabuto's modified reptilian flesh. Despite Suigetsu's vehement opposition to this, Jūgo simply noted that he would carry out Sasuke's will — a will he believed by extension was that of Kimimaro.

With Orochimaru revived, the team headed towards Konohagakure with the Sannin, where they sensed Naruto Uzumaki's chakra leagues away but pressed on nonetheless at Sasuke's behest. Heading silently towards a dilapidated temple, and then back through Konoha, Jūgo used the natural energy from his Sage Transformation on Orochimaru's cue, to activate the spores that were attached to Sasuke, shrinking in the process. Moving to subdue one of the spare clones, Jūgo absorbed the clone regaining his normal size. With this, he looked on as the four previous Hokage that are reincarnated before him. Listening on silently throughout the proceedings, Jūgo is shocked when Tobirama unleashed a swell of killing intent when he heard Sasuke's intentions to exact revenge on Konoha. Moving almost instinctively, Jūgo moved in front of Sasuke until the situation was quelled by Hashirama. When Sasuke decides to aid the Allied Shinobi Forces after hearing Hashirama's story, Jūgo accompanies Sasuke to the area of the battlefield where Sasuke's former team-mates are.

With the situation looking grim as the Alliance was trapped in a barrier with a Tailed Beast Ball-firing tree, Jūgo made his way back over to Sasuke and noted that they needed to get away, and called for Sasuke to use his Summoning Technique to escape. They are, however, stopped by naruto who demands they stand before him so he could give them his chakra. Jūgo later found himself safe and outside the barrier where he questioned what Naruto had done and listened on as it was explained how Naruto and Minato's actions had saved them. Jūgo subsequently watched on as the Shinju attacked and killed the shinobi around them. As Sasuke then began his next attack against Obito, now knowing that the jinchūriki was vulnerable against senjutsu, Jūgo transferred some of his natural energy into Sasuke, who then proceeded to channel it into his complete Susanoo in a manner similar to the Cursed Seal of Heaven.

The combined effort of the Alliance is ultimately enough to defeat Obito. However, they are soon faced with the threat of a revived Madara and Guruguru who assaulted them with a giant wooden statue. Though initially shocked when a distraught Karin lead the charge against the new threat, he joined the fray and was able to catch Karin after she is attacked and removed the stakes from her body as she recovered. As the later raced towards where Sasuke was, Jūgo considered the situation and the fact that Orochimaru might try to take Sasuke's body, apparently ignorant of the fact Orochimaru can only use the Living Corpse Reincarnation once every three years, along the way, Karin detects a new sinister chakra near Sasuke. Upon arriving, they discovered the new chakra signature was from Kabuto, having escaped the loop of Itachi's Izanami. Kabuto plainly stated that he no longer desired Sasuke, rather was attempting to heal him. Orochimaru asks his former assistant why he was doing this, Kabuto explained that his experience under Izanami's power restored his clarity on who he really was, now merely wishing to return to Konohagakure orphange he grew up in and correct his mistakes. Eventually, Kabuto succeeded in healing Sasuke, who quickly returned to his feet and was ready to resume the battle. Jūgo and the others were later caught in the Infinite Tsukuyomi when Madara cast it. However, Jugo and the others were eventually released from Madara's Infinite Tsukyomi by Naruto and Sasuke.

Trivia

 * Jugo can be translated as 15.
 * He can be compared to Freddy Fazbear in several ways:
 * Both are the last ones to make their move.
 * When they become dangerous, the white part of their eyes turn dark.
 * Bears are often called "bipolar" and Jugo is also known as Bipolar Jugo.