Juugo is a shinobi from an unnamed clan who was held at Orochimaru's Northern Hideout. His life is defined by a unique and dangerous physiological condition that causes him to involuntarily absorb natural energy from his surroundings. While this ability, known as Sage Transformation, grants him immense power, it also triggers sporadic, uncontrollable fits of murderous rage. During one such episode, he destroyed an entire village, an act that led others to label him "Juugo of the Scales" or "Bipolar Juugo," referring to his unpredictable, dual nature.

Desperate to prevent himself from harming others, Juugo voluntarily secluded himself in a cave. This self-imprisonment ended with the arrival of Kimimaro, a member of the Kaguya clan. Kimimaro was uniquely able to quell Juugo's violent outbursts, and the two formed a close bond. Following Kimimaro, Juugo surrendered himself to become one of Orochimaru's test subjects, hoping to find a cure for his condition. Instead, Orochimaru became fascinated with the enzymes in Juugo's blood that caused his transformations, using them as the fundamental origin for all of his Cursed Seals. After Kimimaro later died and Orochimaru was defeated, Juugo returned to his seclusion.

Despite his terrifying potential for violence, Juugo's true personality is quiet, gentle, and unassuming. He harbors no genuine desire to kill and is deeply compassionate, particularly towards animals. His murderous impulses are a source of personal torment, and he willingly remains confined to protect others. His central motivation for joining Sasuke Uchiha's team, first called Hebi and later Taka, is to use Sasuke as his "cage". He believes that Sasuke's powerful killer intent and Sharingan are capable of suppressing his rage, just as Kimimaro once did. Learning that Kimimaro sacrificed his life to deliver Sasuke to Orochimaru, Juugo came to believe that Kimimaro's will lives on in Sasuke, earning the avenger his complete and unwavering loyalty.

Within the story, Juugo serves as a powerful but unstable asset to Sasuke's mission, first in the pursuit of Itachi Uchiha and later in conflicts against Killer Bee and the Fourth Raikage during the Five Kage Summit arc. His role is often defined by his immense physical strength and his unique ability to heal others who bear a Cursed Seal, such as Sasuke, by transferring a part of his own flesh to them. This act of healing, however, temporarily reverts him to a younger, child-like form. He can return to his adult state by absorbing the bodies of others.

Juugo's key relationships are few but intense. His bond with Kimimaro is foundational, representing the only friendship and peace he had ever known. He sees Sasuke as the vessel for Kimimaro's spirit and his sole protector against his own inner demons. He has a tense and uneasy alliance with his teammate Suigetsu Hozuki, who is often exasperated by Juugo's mental instability. Karin, another teammate, also recognizes his fragile mental state, viewing him as dangerous and unpredictable.

Throughout his development, Juugo learns to exercise a greater degree of control over his transformations. Unlike those who receive a Cursed Seal, he is not limited to a single form and can perform partial transformations, morphing his limbs into a variety of weapons such as piston-like rams to enhance his punches or massive axe blades. He can also create jet boosters on his back and arms for increased speed and power, or chakra cannons for devastating long-range attacks. In his complete Level 2 state, his skin darkens, his eyes become black and yellow, and he gains immense durability and strength, although he loses his sanity in the process. A more subtle but notable ability is his capacity to communicate with and command animals, which he uses for scouting and gathering information.