Mū

was the of Iwagakure.

Background
During his lifetime, Mū taught Ōnoki how to use Dust Release. At some point in time, both he and Ōnoki met with Madara Uchiha, after an alliance had been formed between their respective villages. However, despite their standing, Madara boldly declared that Konohagakure remained the dominant power and as such, they should obey any orders given. This culminated in both parties engaging in battle where Mū and Ōnoki were defeated.

He also had a great enmity with the Second Mizukage and the two men eventually killed each other in what would be their final confrontation.

Personality
Mū was an extremely calm and collected individual, showing no signs of perturbation to his sudden reincarnation. He's also unfailingly polite, taking his time to recount the previous events to the other Kage upon their forced summoning, and referring to the one whom he both killed and in turn was killed by with an honorific title, apparently bearing no ill-will towards him.

Despite the seeming civility displayed towards his fellow Kage, Mū has demonstrated that he first and foremost prioritises the prosperity of his own village regardless of the manner in which this affluence is achieved, even reminding his disciple to take full advantage of any post-war situation to assure the dominance of Iwagakure by quickly seizing control of the resultant spoils. Even seconds before his sealing he attempted to warn Ōnoki to take precaution. This devious and somewhat merciless trait appears to be just one of the many reasons as to why the Second Mizukage despises him to such an extent. This under-handed tactic would later be taught to his student, which would become the reason for Iwa's bad reputation among the Five Great Shinobi Countries.

Appearance
Mū's entire body is covered by intricately wrapped bandages, with only his violet-coloured eyes and nose being visible, giving him an overall mummy-like appearance. Along with this Mū wears a black headband (dark blue in the anime) with no distinguishing markings and a pair of black pants. He also doesn't wear shoes as his feet are also bandaged heavily. Due to the effects of the Summoning: Impure World Reincarnation technique, he has black sclera and white irides after Kabuto Yakushi eradicates his personality.

When he was still alive, Mū was shown to have donned what appeared to be a standard Iwagakure's flak jacket, with the village distinctive single sleeve outfit over his left arm. He also carried two swords strapped to his back which had slender handles and were also bandaged.

Abilities
Having been the Kage of his village, Mū is an immensely powerful shinobi. He appears to be both extremely knowledgeable and analytical, being able to discern the exact method used to locate his position almost immediately after detection, seemingly without any prior information concerning the technique in question. While alive he carried a pair of what resembles swords wrapped up in bandages.

His skills, especially those with the Dust Release, were such that the Allied Shinobi Forces leaders agreed that the only way to stop him was to have Ōnoki confront him on the battlefield. Mū himself declares that sheer numbers alone do not matter because of the nature of his techniques, in spite of confronting nearly the entire Fourth Division at the time. While Mū is in his split state, he is not able to use Dust Release techniques due to his power being halved.

Invisibility
Mū is capable of becoming perfectly camouflaged with the surrounding environment. His camouflage is so good that he has become renowned as the, 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, Mū considers anyone with the ability to sense his presence worthy of praise.

Nature Transformation
Mū is one of the few people capable of using a kekkei tōta, 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, Ōnoki, 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 Mū's power hadn't waned since his lifetime, a fact later affirmed by Ōnoki.

Other Skills
Mū has displayed a mastery in the ninjutsu field, demonstrating the capability to levitate and fly, as well as being able to split himself into two duplicates to help escape a lethal blow, but at the cost of reducing his power by half. He was also physically strong enough to lift massive rocks off of his body, even with only half of his power.

Mū is also a skilled sensor-type shinobi displaying the ability to sense chakra over a distance of several kilometres, as well as to discern the similarity between the chakra belonging to close relatives such as Gaara and his father, and distinguish chakra from different populations. Furthermore, when these detection capabilities are employed in conjunction with his quick reflexes, he can avoid techniques that even other Kage find difficult to evade, as demonstrated when only he instinctively managed to elude the clutches of Gaara's sand hands and avoid Naruto's attack from behind.

Shinobi World War Arc
In preparation for the Fourth Shinobi World War, Mū was reincarnated by Kabuto Yakushi alongside other deceased Kage, in order to fight against the Allied Shinobi Forces. When they are mobilised, Mū is sent in the direction of the Fourth Division but as he approaches their location invisibly, he is detected by Gaara, something which he commended his opponents for achieving.

Upon being noticed, Mū was forced to summon the Third Raikage, the Fourth Kazekage and the Second Mizukage, who quarrels over the manner of their death. Recognising the means of their reincarnation, Mū searched for the Second Hokage, the technique's inventor and thus the presumed user, but informs his comrades that he could only pick up the presence of someone whose chakra signature was similar to the Fourth Kazekage's and that, that person was backed by a large army. The Kazekage then explained that Tobirama had been dead for many years, whilst declaring that the one known as Orochimaru knew the deceased Hokage's technique as well. Mū later notifies the Mizukage that they're being controlled and thus, some of their movements are restricted, much to the latter and the Raikage's dismay.

Pursuing the division throughout the night alongside the others, he remarks that their indecisive targets had been slowly retreating but that the opposition's movements had now altered, concluding their pursuit. When the Mizukage laments the prospect of confronting shinobi from his own respective village and curses the one responsible for reincarnating them, even Mū approves. He then detects two individuals approaching, who are preceded by a colossal wave of sand that advances towards them, but the Fourth Kazekage swiftly counteracts this with his Gold Dust. At this point, Gaara appears suspended above them and sculpts hands of sand to ensnare the Kage, with only Mū able to escape captivity while sensing Ōnoki's presence above, who appears to have lived a long life thanks in part to his master's teachings. Utilising the same technique to repel that of his former disciple, the converging attacks result in a huge crater in the middle of the battlefield, leading Ōnoki to assert that his predecessor's strength hadn't waned.

Suspecting a shinobi alliance had been formed, due to the accumulation of chakra types from different villages, Mū is still surprised that such a merger has occurred when it is confirmed. Informing his former disciple that only he can stop him, Mū reminds Ōnoki that by taking full advantage of the post-war situation, the dominance of Iwagakure can be assured. Offending the Mizukage, the Raikage and most shockingly Ōnoki with such devious tactics, Mū immediately isolates himself when the battle resumes, pursued only by the latter. As the chase concludes, after seemingly clashing several times, Mū declares to an exhausted Ōnoki, that his age had caught up to him and as he disappears from view, to call Gaara before he died too. However, telling Mū not to underestimate him just yet, Ōnoki creates several clones.

As their battle ensues, Mū takes Ōnoki by surprise from behind in mid-air yet due to Gaara's timely arrival and warning, Ōnoki manages to avoid the blow while seemingly throwing his back out in the process. Preparing to attack them with his Dust Release, Naruto attempts an ambush with his Planetary Rasengan but sensing the attack, Mū easily evades it. Believing his technique to be the faster, only the combined efforts of both Gaara and Naruto ensure that the latter lands a hit on a shocked Mū, sending him hurtling through a nearby rocky outcrop. Colliding with the ground below, Ōnoki swiftly moves in and turns his body to stone, preventing him from moving.

As Mū is being sealed within Gaara's sand, he tells Ōnoki that for someone who detested other villages, he worked well with them, yet Ōnoki replies that he had lived long enough to experience such a change. Just as the seal nears its completion, he says "Be careful, I…" but is sealed in full before he can finish. Sometime later, whilst unearthing himself from the rubble left in the wake of Naruto's devastating attack, a damaged but regenerating Mū, chastises his former disciple for still being naive, having overlooked his body splitting while it was obscured by the prior attack. Unable to remove the seal that bound his other half, Mū is made to await Kabuto's instruction, until the preparations for "that" summoning are finished. After more time passes, Mū performs the Summoning Technique.

Narrowly avoiding the summoned coffin's lid after it’s violently removed, Mū is shocked at the figure who emerges, remarking that the individual behind their revival must have a good understanding of war to summon such a person. Together the pair walked towards Gaara and the Fourth Division, where it is revealed that the one Mū summoned is Madara Uchiha. When Madara notes that all of the shinobi are wearing similarly marked forehead protectors, Mū informs him that an alliance had been formed in response to the on-going war. As Mū is unable to answer Madara's question about who's controlling them, Kabuto erases his personality and uses him to talk directly with Madara. After avoiding Gaara's attack, he attempts to form a Dust Release technique, but it soon disperses due to his power being halved.

Observing the ensuing conflict, Kabuto continues to employ Mū's body to remark upon Madara's demonstration of the Rinnegan, even after the impact of two enormous meteorites. Reforming in the wake of the collision, Kabuto has Mū approach the few remaining survivors, whilst providing Madara with a detailed overview of current affairs. However, when the two resume their attack, they are repelled by Tsunade and A who had just been transported to the field. Recovering from A's punch, Mū is pursued by Dodai and the Hokage Guard Platoon. As Mei creates a mist, obscuring the battlefield, Kabuto positions Mū right in front of Madara but he is easily knocked away by a lightened A.

Once recovered, Mū eludes his pursuers under Kabuto's direction, as the battle between Madara and the Kage intensifies. With the Impure World Reincarnation being released, Mū's consciousness returns to him but only for a short while as he in enveloped in a light of sorts and begins deconstructing, much to his shock.

Trivia

 * can mean 'nothingness' or 'non-existence', possibly alluding to his ability to become invisible.
 * His combination of bandage-covering and the technique he uses may be referring to Griffin, the main character of "The Invisible Man" novel of H.G. Wells.