Mu is the Gold Saint of Aries, and as such guards the first of the twelve Zodiac temples of Sanctuary. He is native to the Asian country of Jamir like his master Aries Shion, who later became Grand Pope. Mu is the only one capable of repairing the Saint Cloths.
Mu's attacks include "Starlight Extinction", "Stardust Revolution", and "Crystal Wall". He also possesses formidable telekinetic powers, an ability that is unique among the Saints. Like the other Gold Saints, he is also capable of telepathy and limited healing. Mu's apprentice is a young telekinetic boy by the name of Kiki.
During the Sanctuary arc, Mu passively rebelled against the false Pope, and stayed away from Sanctuary, preferring to reside in Jamir. He appeared at the Rozan Five Old Peaks on one occasion and intervened when Cancer Deathmask had come to kill Roshi. Rather than fighting the Bronze Saints who reached his temple, he repaired their Cloths for them.
Upon the Specters' invasion of Sanctuary, Mu defeated several of them, most notably Papillon Myu. Mu fought against the resurrected Gold Saints alongside Scorpio and Leo, with whom he performed the forbidden Athena Exclamation technique. He then traveled to the Underworld to aid Athena, but was quickly defeated by Wyvern Rhadamanthys, as a result of his power being drained by Hades' realm. Mu was revived from Cocytus and joined the other Gold Saints in their destruction of the Wailing Wall.