Muu was the Second Tsuchikage of Iwagakure, the Hidden Rock village, and a powerful shinobi who fought and killed the Second Mizukage in battle at the cost of his own life. During his life, Muu was the teacher of Onoki to whom he taught the secrets of the Dust release.
Now he has been resurrected by Kabuto's Summoning: Impure World Resurrection technique and is bound to his will. The purpose of his resurrection was to fight in the Fourth Shinobi War against the Allied Shinobi Forces.
He tried sneaking behind enemy lines, but was detected by Gaara's sand. Spotted, Kabuto used him as a proxy to summon the 2nd Mizukage, 3rd Raikage, and 4th Kazekage to reinforce him. Together they entered battle against forces lead by Gaara and Onoki, the Mizukage and Raikage dealt with the soldiers while the Kazekage fought Gaara and Muu fought Onoki; a battle Onoki was quickly losing. After Gaara managed to subdue the Kazekage, he immediately assisted Onoki against Muu but were still losing. It wasn't until Naruto using his newfound Kyuubi mastery that together they were able to defeat Muu.
Muu tried to warn them of something before he was sealed, after they left it was revealed that they only sealed a duplicate and Muu is about to continue the fight.
Muu's clothing style was pretty unusual, his whole body was covered in bandages, with only his eyes and nose visible. When he was still alive, he used to wear a standard Jonin uniform from Iwagakure and had two swords strapped to his back.
Powers and Abilities
Being a Tsuchikage of his village, Muu was a really powerful shinobi. He was very skilled sensor type shinobi, being able to sense chakra from over a couple kilometers away. In the past he used to carry a pair of swords on his back which would indicate that he was very skilled with them. Another one of his abilities was his perfect camouflage technique which allowed him to become completely invisible and even undetectable by other sensor type shinobi. This ability gained him the title ''The Invisible Man'' and ''Non-Person''. But perhaps his most powerful ability was his advanced Bloodline Limit : Dust release. This allowed it's possessor to combine earth, wind, and fire elements to destroy matter on a molecular level. Muu was so feared for his Dust Release that even the Allied Shinobi Forces said that only Onoki, his student, could stand against him. Like his student, he has displayed the ability to fly. He can also split his body to make a perfect duplicate of himself, however both halves only have half the power.