|Novel||Kakashi Hiden: Lightning in the Icy Sky (Mentioned)|
|Movie||Naruto the Movie: Blood Prison|
|Appears in||Novel, Movie|
Mui's wife passed away not long after their son, Muku, was born, so he raised him all by himself. He was a caring father who loved his son deeply, often taking Muku to eat ramen. Ten years ago, as the guardian of the Box of Ultimate Bliss, Mui, tried opening it once with Muku's chakra, in the hopes of asking the box to grant them the wish of reviving Kusagakure to its former glory. However, Muku didn't have enough chakra and was sucked into the box instead, leaving Mui grief-stricken. As a result, he sought a way to reopen the box to ask for his son back, unbeknownst to his superiors.
Mui was a very strict and solemn man. He took his duties very seriously and followed the rules to the letter. He hardly showed any emotions outside of anger when prisoners tried to defy him for example and was not hesitant in punishing them in the most cruel of ways. While Ryūzetsu and the four council members witnessed Mui showing no outward emotions when he sent Muku into the Box of Ultimate Bliss, which led them to believe he is a cruel and heartless man who would sacrifice anyone for the sake of Kusagakure, Mui is, in truth, hiding his true emotions and the opposite of what they believed: he would forsake his village's welfare for his son. When his son was released from the Box of Ultimate Bliss, Mui showed a softer side to himself and even shed tears at the prospect of seeing his son again. In a flashback it was revealed that Mui wasn't seemingly always as hardened as he was and seemed to care greatly for both his wife and son.
Mui had long black hair that goes beyond his shoulders, though ten years ago, when he was trying to use Muku to open the Box of Ultimate Bliss, his hair was around shoulder-length. He wore some sort of suit with a brown belt and a silver buckle. He also wore brown boots. When meeting with other members of the Flower Group, he donned the metal head of a bull and wore loose-fitting, black robes.
Being the warden of the Hōzuki Castle as well as a jōnin, Mui was a highly competent and skilled shinobi. Mui had shown great skill with taijutsu, being able to defeat multiple prisoners barehanded without losing his composure, as well as easily defeat a weakened Naruto with a series of swift punches. He also showed an incredibly level of resilience, as even after suffering a mortal wound through the chest by his son, Mui managed to get back up on his feet and apply a powerful seal. Mui also has knowledge of pressure points, knocking Naruto unconscious with a few strikes.
Mui had demonstrated great proficiency with Fire Release. The most prominent of his fire techniques was one which can bind a prisoner and prevent them from moulding chakra while at the same time absorbs their chakra after he applies a formula to them, it will also set them aflame should they try to escape. He also had an advanced form of this which he could use on larger targets. By altering this technique, he could surround his arm with fire to use as a blade which will engulf those stabbed in flames, subsequently setting his opponents ablaze.
Mui makes his début, informing the new set of prisoners about the Hōzuki Castle. When Naruto Uzumaki voiced his innocence, Mui forcefully branded him with the Fire Release: Heavenly Prison technique telling him that proving his innocence was of no concern to him as his village had deemed him a criminal and it was his duty to contain him. Mui later has Maroi kidnap Naruto to be used in his experiments but it is revealed to be a clone. The real Naruto is later captured however, and placed in solitary confinement. After meeting with other members of the Flower Group he discusses the fact that they had Konohagakure's jinchūriki in their possession and that he could be used to open the Box of Ultimate Bliss as well as Kazan who had been plaguing them for some time. After much efforts and quelling several rows that broke out in the prison subsequently, Mui was finally able to seemingly kill Kazan on a cliff-side and get the real Naruto into their lab. Siphoning off his chakra and feeding it to the box, they are finally able to awaken the Box of Ultimate Bliss.
Believing Mui to want to ask the box to return Kusagakure to its former glory, the other members look on with joy and anticipation in their voice only to hear Mui tearfully ask the box to return his son, Muku, to him who was sealed within it the last time it was opened. As they try to stop him, Kazan, who is revealed to be alive, stopped them. When his son emerged from the box, Mui wept with joy before his son ran his hand through his chest seriously wounding him before turning into Satori. It was through Naruto's words that gave Mui the strength to get up and attack Satori with the Fire Release: Great Fire Heavenly Prison technique which suppressed it enough for his son to revert to his human form. As his son looked at him, Mui is attacked once more by Muku and his body is sent flying to the mural that he built for his wife where he dies.
After Mui's demise, Hōzuki Castle was left without a warden. His position was eventually filled after the Fourth Shinobi World War, when Kahyō was given a deal to become the new warden with her Ice Release: Earthen Consecutive Chains of Ice becoming the new sealing shackles for the prisoners, in exchange for avoiding a death penalty.
- His name means "inactivity".
- (To Naruto) "Have you ever killed someone you had no grudge against in order to protect your village?"
- (When wishing to the Box of Ultimate Bliss) "Kusagakure has been dead to me for a long time. Ever since I lost Muku ten years ago!"
- (To Muku from afar) "Let us at least die together, Muku. This is all I can do to atone for my sins!"