Juugo is the means behind Orochimaru's development of the cursed seals. Some time in the past, Juugo purposely sought out Orochimaru to cure him of his "impulses". Juugo 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 Juugo'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 Juugo initially appears quite bloodthirsty, Karin notes that this is only common behavior for the enraged; Juugo himself doesn't want to kill anyone. Juugo'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. Before Sasuke, it was Kimimaro who kept Juugo 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, Juugo believes that Kimimaro's spirit continues to live on in Sasuke.
Juugo 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 Juugo 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.
While discussing Sasuke's team with the other members of Akatsuki, the Akatsuki leader refers to Juugo as Juugo of the Scales (天秤の重吾, Tenbin no Jūgo?, "scale" referring to something that determines balance). Whether this is a title or a location is unclear.
After Team Snake is dispatched on their mission into the forest, Jugo commences his search by activating his special enzyme and asking a bird for information.