Manga Debut: Chapter #515

Anime Debut: Naruto Shippūden Episode #256

Muu (無, Mū) was the Second Tsuchikage (二代目土影, Nidaime Tsuchikage; Literally meaning "Second Earth Shadow") of Iwagakure.

Muu is capable of becoming perfectly camouflaged with the surrounding environment. His camouflage is so good that he has become renowned as the "Non-Person" (無人, Mujin), a title which he gained for seemingly having neither form nor chakra, while the technique was active. It is impossible to detect him by normal means and so, Muu considers anyone with the ability to sense his presence worthy of praise.

Muu is one of the few people capable of using a kekkei touta, an advanced and extremely powerful form of kekkei genkai. In particular, he possesses the Dust Release, which allows him to combine the fire, earth and wind natures to reduce his opponents to nothing more than mere dust, destroying them down to even a molecular level. Compared with fellow dust-user, Ounoki, his power is implied to be on equal terms, as when the two initiated Dust Release: Detachment of the Primitive World Technique, the disastrous outcome although chaotic, demonstrated no evident victor in regards to strength. This may be due to the fact that Muu's power hadn't waned since his lifetime, a fact later affirmed by Ounoki.

(Source: Narutopedia)