Jūgo

History
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.

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.

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.

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. 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. Finally, Jugo is able to understand birds, using them as scouts and to help him accomplish tasks.