|Movie||Naruto the Movie: Blood Prison|
When he was an infant, Muku lost his mother and was raised by his father, Mui, the guardian of the Box of Ultimate Bliss. He was regarded as a prodigy of his village and, therefore, his chakra was chosen to open the box in hopes of returning Kusagakure to its former glory. The day before it happened, Muku promised his best friend, Ryūzetsu, that he would return no matter what happened. Muku's chakra wasn't enough and he was sucked into the box instead where he spent ten years trapped in despair and grief, with all his attempts to break the box from the inside failed.
As a child, Muku was a cheerful individual that loved his father dearly. His greatest wish was to live a peaceful life with his father and his best friend. However, after spending years inside the Box of Ultimate Bliss, he reemerged as an apathetic, cold and cruel person, if not bearing a grudge against his father for feeding him to the box; this is shown when Muku was not hesitating in the slightest to run his hand through his father's chest.
Muku had pale skin which contrasted with his jet-black hair and grey eyes. He wore a blue kimono shirt with the sleeves rolled up, a scarf around his neck and black arm-warmers. He also wore a pair of blue pants and sandals on his feet. After emerging from the Box of Ultimate Bliss, he was noticeably taller. His skin became extremely pale, his hair was longer and had dulled in colour and he was wearing the same clothes as he had when the box was open, which has since been in tatters.
Muku was stated to be a prodigy of his village. He was very proficient with Fire Release techniques, allowing him to make fire-like apparitions to attack his enemies with Fire Release: Demon Lantern. He has also been seen to be strong enough to pierce his father's chest and bisect Kazan at the waist barehanded. He was knowledgeable, as he knew of Naruto's Sage Mode, and was said to have a great amount of chakra, as his father used him in an attempt to open the Box of Ultimate Bliss, though it was not enough to accomplish the feat.
Playing host to the demonic puppet Satori granted Muku the ability to sense his enemies' bloodlust, which, when combined with the newly gained wings and agility, allowed him to easily evade all attacks. He can also launch deadly energy spheres and feathers at high speed to pierce his targets. At his last moments, Muku showed a variant of shooting feathers from his mouth by screeching that not only killed his father but also send him flying away.
As a young man, Muku was sealed into the Box of Ultimate Bliss as a sacrifice in order for his father to open it. This was done in the hopes of asking the box to grant them the wish of reviving Kusagakure to its former glory. Grief-stricken by having to do this, Mui sought for a long time to reopen the box to ask for his son back and when this day came to pass, Muku emerged from the box as an adult. Happy for getting his son back and feeling guilty, Mui asked for Muku's forgiveness. Muku, however, stabbed his father straight through the chest with his bare hands and then killed Kazan when he came to Mui's aide, before turning to his fallen father and telling that he should have never brought him back.
After seeing his friend Ryūzetsu again he transforms into Satori who begins to go on a rampage. Naruto Uzumaki and Gamabunta fight him, but Satori's ability to sense fear and killing intent allows Muku to have the upper hand. Even with the combined forces of Killer B, the Konoha 11, Sai, Yamato, Kakashi and Might Guy, Muku evades all of their attacks. Only when Naruto realises that Satori can't sense Ryūzetsu because she has no fear, he is able to hit Muku and tries to make him understand that it wasn't through severing his bonds that he could be strong and shouts at him for attacking his father and being unable to see his pain or hear Ryūzetsu's pleas. Muku then impales both Naruto and Ryūzetsu, who jumped in front of Naruto. However, Naruto is able restrain the beast using two clones and urges Mui's help who brands Satori with the Fire Release: Great Fire Heavenly Prison technique. Naruto then attacks Satori with his Wind Release: Rasenshuriken.
After reverting to a human form, Muku, seemingly regaining some sense of himself, acknowledges Ryūzetsu and grabs his fathers hand, telling him that they're still big. While Mui cries, Muku opens his mouth and unleashes a final attack on his father, killing him before activating the Fire Release: Demon Lantern and engulfing himself in flames. He apologises to Ryūzetsu for being unable to keep the promise he made to her to return no matter what and states that at least now, he and his father could rest peacefully. As he is engulfed in flames, his body is incinerated and Satori returns to the box which closes.
Moments later, countless fireballs rise all around the castle, hinting that Muku has died and his spirit was finally freed from the box. Ryūzetsu catches one of the fireballs and welcomes Muku home, saying that it took him quite a while.
- Muku's name means "purity" or "innocence".
- Along with Ryūzetsu, his Kusagakure forehead protector had four blades of grass, instead of the normal three.