Shinnou

Shinnō initially appears compassionate, knowledgeable, and committed to medicine. His skill and apparent willingness to risk himself for patients make him deeply respected by Amaru and her village.

That persona is a deliberate deception. Shinnō is arrogant, manipulative, and fascinated by using human suffering to generate the negative emotions required for dark chakra.

Shinnō possesses extraordinary medical knowledge, including advanced regeneration techniques. His Body Revival Technique manipulates his physical condition and allows him to gain extreme muscular power through dark chakra.

He understands the Eight Gates and possesses enough knowledge of forbidden medical techniques to be of interest even to Orochimaru. He can also merge with the Zero-Tails and draw upon the creature's dark chakra reserves.

Shinnō arrives in Konoha with Amaru after the Sky Ninja attack and joins the mission back to her village.

He eventually reveals himself as an operative of the Land of the Sky who has spent years cultivating negative emotions to empower the Zero-Tails and Ancor Vantian. Sasuke disrupts his Body Revival Technique, and Naruto and Sasuke later overwhelm the Zero-Tails system sustaining his plan.

Amaru is Shinnō's apprentice and the person he manipulates most intimately. Naruto Uzumaki opposes him after discovering the truth.

Sasuke Uchiha confronts Shinnō while carrying out an assignment for Orochimaru, who previously learned regeneration-related knowledge from him.