Muku was a shinobi from Kusagakure appearing in the Naruto: Shippuden film Naruto the Movie: Blood Prison. He was the son of the Hozuki Castle prison warden, Mui. He was regarded as a prodigy of his village, before being sacrificed by his father to open the Box of Ultimate Bliss, that was said to grant any wish and had once allowed Kusagakure to nearly conquer the world. Muku's chakra was insufficient and he was absorbed and trapped inside the box instead.
Before risking himself for the opening of the box, he promised his best friend Ryūzetsu that he would come back safely. Later, he was freed from the box by his father. The Box of Ultimate Bliss corrupted him causing him to pierce his arm through Mui's chest and then transforming into Satori.
He was turned back to normal by the combined efforts of Naruto and his jaded father. He used his Fire Release: Demon Lantern jutsu in order to activate the seal his father placed on him, killing himself before Satori regain control of his body. His father was killed when Satori tried to regain control. Before he died, he apologizes to Ryuzetsu for not keeping his promise and that at least he and his father can rest peacefully.
Muku had black hair, gray eyes, and pale, white skin. Before being sent to his doom inside the Box of Ultimate Bliss, he wore a sleeveless light blue kimono with black arm-warmers and a pale green scarf that covered most of his neck. The area of the scarf above the left side of his chest appeared to be a vertical line of much paler shade and about as wide as his arm, almost white. He seemed to wear light gray pants and sandals.
After emerging from the box, his clothes were in tatters, with half of his chest and upper abdomen exposed, torn pants, and no longer having a scarf. His skin had become extremely pale.
After impaling his father without a second thought shortly after his revival, he turned into Satori, an odd-looking monstrous bird that stood on two feet and had a torso as skinny as his spine. His shoulders were on top of his head and were connected to arms with very large talons that appeared to function as hands. His large pair of wings were also connected to his eyeless head. The head had a mouth that was almost as wide as its front, and had herbivore teeth with four ridiculously long teeth that were sharp and curled like husks. It could even be said that it doesn't have a head but it's chest has a mouth and its torso is his abdomen. Overall, the demonic bird has a rather strange and menacing appearance, being several meters taller than Gamabunta, who is roughly 30 meters tall.
Muku had a normal personality for a child, cheerful and appreciative towards his family. However, during his years spent sealed inside the Box of Ultimate Bliss, he had a mental battle with the dark form known as Satori. He reemerged as an apathetic, cold, and cruel individual who loathed his father for sacrificing him to the box. His evil personality was formed under the influence of Sasori, and wasn't entirely his. After impaling his father without a second thought shortly after his revival, he turned into Satori. During his rampage, his childhood friend Ryūzetsu tried to bring back Muku's real personality. After reverting to his human form near the end of the movie, he incinerates himself with his own jutsu and apologizes to her for being unable to keep his promise to come back alive. His final wish was for the three of them to rest in peace.