Mu has purple hair and gray eyes. His traditional outfit without his Cloth consist of a yellow top, with a red sash draped over his shoulders, and a white belt. This outfit is replaced by a green outfit as he wears his Cloth.
Mu is a very gentle Saint, showing much concern for his Bronze and Gold Saint allies, often offering his support to the Bronze Saints as they work against the Pope and the Sanctuary.
He is also shown to be a formidable opponent if he really gives everything, as seen when he fights Deathmask and Aphrodite when they return as Specters.
Mu first appears at the Jamir Mountain to answer Shiryu's call to restore his and Seiya's Bronze Cloths who have been damaged. Mu states that the Cloths are dead, but he can repair them if Shiryu gives a third of his blood, which will be dangerous for Shiryu.
The Bronze Saint still insist on repairing them, and willingly offers his blood. Mu, in amazement of Shiryu's act, decides to fix the Cloths, allowing Seiya and Shiryu to fight alongside Shun and Hyoga against Ikki and the Black Saints.
Mu returns to interrupt Deathmask's attempt at Dohko's life, revealing his status as a Gold Saint and saving Shiryu from defeat. He persuades Deathmask to return to Sanctuary, also mocking Deathmask about using his techniques against a Bronze Saint.
As Deathmask leaves, Mu remains with Dohko, Shiryu, Kiki and Shunrei as Dohko officialy counts Shiryu as one of Athena's Saints and comrade, rather than just a student.
The battle of the twelve Houses
House of Aries
Mu reappears again as Seiya, Shun, Shiryu and Hyoga make way to the House of Aries after Ptolemy shot Athena with the "Phantom Arrow". As the Bronze Saints move on, they are suddenly attacked by a large, falling rock. As they evade, they see Mu emerging from the House of Aries, and he asks what they will do if he is an obstacle to them. Shiryu, disappointed by Mu's words, charges at the Gold Saint, but it easily defeated.
It is revealed that Mu only wanted to show them how damaged their Cloths were from their countless battles against the Silver Saints. He offers to repair them, although it will require an hour. As he repairs the Cloths, he also informs the Bronze Saints of the 7th sense, which will allow them to rival the Gold Saints in power, or they won't stand a chance in saving Athena at all.
During the rest of the Bronze Saints' battle against the Gold Saints, Mu watches over Athena until the other Bronze Saints Jabu, Ban, Geki, Ichi and Nachi arrive along with Tatsumi to defend her.
Statue of Athena
As the battle reaches it's climax with Seiya and Ikki fighting Saga, Athena arrives at the House of Aries, and Mu accompany her and the Bronze Saints through the rest of the Houses, meeting up with the surviving Gold Saints before arriving at the scene of the final battle, where they witness Seiya burning his Cosmo, unleashing a Big Bang as he destroys Saga's evil side, before Saga takes his own life at Athena's staff, ending the Sanctuary battle.
Mu appears along with the other Gold Sains as they stand in the rain, aware that Seiya and his friends are fighting the Marina Generals and Poseidon in order to rescue Athena from the Main Breadwinner. Aioria insists that he will go to help Seiya defeat Poseidon, but Mu strictly says that they must remain at Sanctuary to prepare for the war that is about to come, as instructed by Dohko. They rejoice as they see the raining ending, and the sun emerging again, meaning that Poseidon has been sealed once more.
House of Aries
Mu is given strict orders to get rid of Seiya or any of the other Bronze Saints by Athena, should they arrive at the Sanctuary, as she did not want them to get further hurt, due to the several battles they have suffered already. As Deathmask and Aphrodite reveal themselves as newly resurrected Specters, Seiya arrives to assist Mu when he is attacked by Deathmask.
Before Seiya is able to resume his battle against the former Gold Saint, Mu suddenly strikes Seiya from the back, before attacking him with the "Starlight Extinction", sending Seiya away.
As the former Gold Saints try to break through, Mu unleash his true power, intending to kill them, and sends them to Yomotsu Hirasaka, where they are killed by Rhadamanthys. As Saga, Shura and Camus also reveal themselves, Mu is restrained by a mysterious figure observing, allowing the three former Gold Saints passage. He pursues them as soon as Dohko arrives to fight the mysterious man, revealed to be Shion, Mu's mentor and former Pope.
House of Taurus
Mu arrives at the Taurus Temple where he notices Aldebaran, shortly before realising that he is actually dead. As the Cloth disassembles, Mu finds himself confronted by Niobe, one of the true Specters and Aldebaran's true murderer. Realising that he has already been killed by Aldebaran's "Great Horn", Mu quietly walks away, as Niobe is torn apart by the late effect of the attack.
House of Cancer
Arriving at Cancer Temple, he is confronted by a group of Specters, and is also surprised to see Seiya once more, as he challenges one of the Specters, ending up defeated easily. However, a mysterious voice interrupts their battle, stating that he wish to kill Mu himself. Sending the others onward, the voice reveals himself as Myu, a Specter mastering psykokinesis. As Seiya challenges him, Myu and Mu are shocked to see Seiya managing to withstand Myu's psychokinesis which was earlier able to restrain a Gold Saint. Sending Seiya onward, Mu switfly deals with Myu, killing him before moving onward.
House of Virgo
As Seiya, Shun, Shiryu, Hyoga and Aioria arrive at the Virgo Temple, they find Mu standing in front of the door leading to the Twin Sala Gardens, refusing to let anyone in, as Shaka has prepared himself to die at the hands of Saga, Shura and Camus (the true intention being to awaken the 8th sense and travel to the realm of Hades).
As he die, the former Gold Saints open the door from the gardens, and after the arrival of Miro, the Gold Saints and the former Gold Saints clash in a massive battle using "Athena Exclamation" on each other. The energy is disrupted by Seiya, Shun, Hyoga and Shun who burn their Cosmo beyond the Seventh Sense, making sure that the energy doesn't destroy Sanctuary. The Gold Saints later bring the former Gold Saints to Athena's Temple, where Athena commits suicide, also awakening her 8th sense.
Mu, Aioria and Miro arrive at Hades' castle, where they fight off some Skeletons before facing Rhadamanthys, who easily gains the upper hand against the Gold Saints. Even the arrival of the Bronze Saints in their new Bronze Cloths isn't enough to ensure the victory against one of the three Judges, so Mu, Miro and Aioria sacrifice themselves in order to let the Bronze Sainst pass, entrusting Athena's care to them.
They are given life again thanks to Athena's divine Cosmo, and they easily dispose of the Specters nearby, before making their way to Giudecca in order to rendezvous with the other Gold Saints. As all twelve Gold Saints are gathered, they combine their Cosmo into Aioros' arrow, and launch a destructive blast at the Wall of Suffering, which manage to destroy the wall, but at the cost of all 12 Gold Saints' lives.
Being a Gold Saint, Mu is able to move at the speed of light, launching attacks at 300 000 km/s. Like his fellow Gold Saint comrades, his Cosmo has a powerful golden aura.
Crystal Wall: Mu creates a large wall which is able to deflect any attack directed at it. Only one other person except Mu is able to remove the wall, that being his mentor, Shion.
Stardust Revolution: Mu gathers his Cosmo into millions of star particles, launching them at his opponents at the speed of light.
Starlight Extinction: Mu's most powerful attack. He gathers his Cosmo and sends his enemies into another dimension or other place, depending on Mu's wish for his opponent.
Cloth Repairer: Mu has the ability to repair other Saint's Cloth with Stardust Sand and Gammanium, the materials needed to repair the Cloths, as well as the Blood of a Saint, in order to give the Cloth its life back.
Mu wears the Aries Gold Cloth, one of the twelve most powerful Cloths which has been bathed in sunlight. It resembles a mighty ram, with large, golden horns at his shoulders. The Cloth covers most of his body, leaving little of his body unprotected. The Cloth also feature a white and light blue fabric cape draped around his shoulders.