Jūgo

Jugo was being held at Orochimaru's North Base, which is also where most of Orochimaru's secret research was conducted. Suigetsu can't stand Jugo and comments that one of the reasons he had to retrieve Zabuza Momochi's sword was because of him. Karin notes that Jugo is mentally unstable, a view confirmed by images of Jugo muttering to himself over who he will kill next, which he determines by gender sometimes. While discussing Sasuke's team with the other members of Akatsuki, Pein refers to Jugo as Jugo of Tenpin (天秤の重吾 Tenpin no Jugo literally "Jugo of the Scales"). Whether this refers to his family or clan name or his place of origin is unknown.

Background
Jugo is the means behind Orochimaru's development of the cursed seals. Some time in the past, Jugo purposely sought out Orochimaru to cure him of his "impulses". Jugo has trouble controlling his rage, which unexpectedly causes him to transform into a berserk demon (much like a cursed seal does to its user). Upon examination, Orochimaru was able to isolate enzymes in Jugo's blood as the cause of this change. By extracting these enzymes, Orochimaru was able to use them to produce similar, but more controlled, effects in other people.

Appearance
Jugo has a bracket-shaped symbol on his left palm. Whether or not this is also present on his right palm or if it has any significance has yet to be seen.

Personality
While Jugo initially appears quite bloodthirsty, Karin notes that this is only common behavior when he is enraged; Jugo himself doesn't want to kill anyone. Jugo's fits of rage are random and thus the reason why he willingly remains confined. The only reason he agrees to go with Sasuke is because he views Sasuke as the only person capable of preventing him from killing people while enraged, by the means of killer intent or the Sharingan. Before Sasuke, it was Kimimaro who kept Jugo in check and as Kimimaro was the only person Jugo came into any contact with, the two grew close. After hearing that Kimimaro sacrificed his life to deliver Sasuke to Orochimaru, Jugo believes that Kimimaro's spirit continues to live on in Sasuke.

Joining Team Snake
Following his defeat of Orchimaru and recruitment of Suigetsu and Karin, Sasuke Uchiha traveled to Orchimaru's North base to recruit Jugo for his immense power. Initially unwilling to join forces with Sasuke, Jugo tapped into his Cursed Seal, but was then frightened into releasing it by Sasuke's killer instinct. He then joins the newly formed Team Snake in their mission to hunt down and destroy Itachi Uchiha, Sasuke's brother. Following an encounter with Deidara of the Akatsuki, Jugo discovered a hidden base belonging to the group where Sasuke confronts a clone of Itachi, who challenged Sasuke to meet him at the Uchia Clan Stronghold. Jugo remained behind with the other members of Team Snake with Kisame Hoshigaki as Sasuke confronted Itachi. After Itachi's death and Sasuke's "education" in his brother's true past by Madara Uchiha, Jugo and the other members of Team Snake were reformed into Team Hawk.

Team Hawk
Having allied themselves with the Akatsuki, Team Hawk was sent to capture the Eight-Tailed Beast so that it could be extracted from its host, Kira Hachi. Infiltrating the Land of Thunder and the Kumogakure, they confronted Kira only to have him defeat Suigestu. When Jugo attempted to defeat him as well, Kira overpowered him using the sword Suigetsu had wielded, which originally belonged to Zabuza Momochi.

Abilities
His brief fight with Sasuke and Suigetsu revealed that Jugo can control his transformation, unlike the static effects of a standard cursed seal. When battling Sasuke he changes his left arm into a piston-like ram to increase his punching power and then alters it into a massive blade from his forearm to fight Suigetsu. He is also capable of doing only a partial transformation, invoking a level two form over one portion of his body without completely changing. Also since Jugo is the very creator of the curse mark, he is able to use it to full extent and it will not erode his body. Finally, Jugo is able to understand birds, using them as scouts and to help him accomplish tasks.