Juugo, naturally nice and gentle, but has homicidal urges due to the Curse Mark. He sought Orochimaru to cure him and was locked away, Orochimaru instead found a way to give others the Curse Mark.
Juugo feels that he needs to be locked away and went to Orochimaru to cure him, but whether Orochimaru didn't or couldn't is unknown. He was kept in a prison lab, bound and locked up. He was befriended by Kimimaro, one of Orochimaru's most trusted servants. Juugo originally saw Kimimaro as a copy-cat, seeing as he was given a version of his curse mark, but Kimimaro told Juugo that he was dying and that it was Juugo's strength that was keeping him alive. Kimimaro seemed to be able to calm him down and he was sad when he found out Kimimaro's body finally gave out trying to ensure Sasuke's retrieval.
Years later when Sasuke freed Juugo, he was initially in his berserker state and Suigetsu fought him off, but Sasuke stopped them. Since Sasuke was important to Kimimaro, Juugo calmed down and decided that he would help Sasuke. But the threat of him losing control scares him and he hopes Sasuke will be able to keep him in check.
Though Juugo has yet to truly show his abilities, it is known that his curse increases his strength and chakra, it also allows him to transform parts of his body into weapons, such as turning the back of his forearm into a large axe. He has also turned his arm into a type of jackhammer and formed a jetpack from his back. He also seems to be able to communicate with animals.