When he was just a child, Muku's mother passed away and he was raised by his father Mui, who was the guardian of the Box of Ultimate Bliss. Muku was a prodigy and his chakra was chosen to open the box in hopes that Kusagakure could return back to its glory. The day before this happened, Muku promised his best friend Ryuzetsu that he would come back no matter what happened. Muku's chakra wasn't enough and he was sucked into the box where he spent ten years in grief and pain, as all his attempts to escape had failed.

Mui tried his best to reopen the box to get his son back. When Muku finally came out of the box as an adult, Mui was really happy but also felt very guilty. Mui asked for Muku’s forgiveness, but Muku stabbed his father straight through the chest with his bare hands and then killed Kizan when he came to help Mui. He told his dying father that he should have never brought him back.

After seeing Ryuzetsu again, he transforms into Satori who begins to rampage. Naruto and Gamabunta fight him but Satori's abilities to sense fear and killing intent allows Muku to have the upper hand. Even with Konoha's combined forces, Muku easily dodges their attacks, but only then did Naruto realize that Satori couldn’t sense Ryuzetsu.

This was because she did not have any fear. Naruto uses this to hit Muku and tries to reason with him, saying that severing his bonds did not make him strong. Naruto shouts at him for killing his father and disregarding Ryuzetsu's pain. Muku impales both Naruto and Ryuzetsu but Naruto is able to restrain the beast using two clones. He asks for Mui's help who brands Satori with Fire Release: Great Fore Heavenly Prison. Naruto then attacks Satori with Rasenshuriken.

After turning back into a human, Muku regains a sense of self and acknowledges Ryuzetsu and grabs his father's hand. While Mui cries, Muku unleashes a final attack on his father which kills him, before activating the Fire Release: Demon Lantern and engulfing himself in flames. He apologizes to Ryuzetsu for being unable to keep the promise he made to her to return no matter what and says that he and his father could now rest in peace as he gets engulfed in flames. His body is incinerated and Satori returns back into the box.

As a child, Muku was a cheerful person who loved his father a lot. He wished to live a peaceful life with his father and his best friend Ryuzetsu. However, after spending 10 years inside the Box of Ultimate Bliss, he became an apathetic, cold and cruel person. He also held a grudge against his father for feeding him to the box.

It was later on revealed that this was due to Satori's influence and the real Muku was still inside him, as he was able to resist Satori after his father sealed him. Even though he ruthlessly killed his father, Muku still had the strength to attack the Box of Ultimate Bliss and apologized to Ryuzetsu for being unable to keep his promise. He said that he and his father could now rest peacefully before sacrificing himself.

Muku had pale skin, jet black hair and gray eyes. He wore a blue kimono shirt with rolled up sleeves, a scarf, and black arm warmers. He also wore a pair of blue pants with sandals. After coming out of the Box of Ultimate Bliss, he had grown taller. His skin had become paler, his hair was longer and dull in color and he was wearing the same clothes which he had worn before, but they were in tatters.

Muku was considered as the prodigy of his village and was very proficient in Fire Release. He made fire-like operations to attack his Enemies with Fire Release: Demon Lantern. He was also strong enough to pierce his father's chest and bisect Kazan at the waist level barehanded. He was knowledgeable as he knew about Naruto's Sage mode and had a great amount of chakra as his father had used him to open the Box of Ultimate Bliss, though it was not enough to complete the task.

When he was Satori’s host, Muku had the ability to sense his enemies' bloodlust and when it was combined with the newly gained wings and agility, it allowed him to easily evade all attacks. He could launch deadly energy spheres and feathers at high speed to pierce his targets. In his last moments, Muku showed a variant of shooting feathers from his mouth by screeching that not only killed his father but also sent him flying away.

• "Sorry. I couldn't keep my promise to you, Ryūzetsu… This time, with father and you… we'll truly live here peacefully together."

Q. What does Muku mean?

A. Muku’s name translates to 'purity' and 'innocence'.

Q. Where was Muku from?

A. Muku belonged to Kusagakure, the Village of the Hidden Grass.