- Age: 20
- Date of birth: March 27
- Nationality: Tibet
- Rank: Gold
- Height: 1.82 m
- Weight: 75 kg
- Blood type: A
- Techniques: Glamour Illusion, Crystal Wall, Crystal Net, Starlight Extinction, and Stardust Revolution
- First appearance: Manga: chapter 12, volume 3 - Anime: episode 10
Under the tutelage of Shion, the Pope of the Sanctuary, Mu was able to become the Gold Saint of Aries at a young age. While all gold saints have a base of psychic powers to some degree, Mu has been stated to possess the strongest raw psychic talent among all living gold saints. This allows him to manipulate his gift to various degrees: psychokinesis, telepathy, teleportation, and various other powers are at his command.
Mu is very soft spoken, with a scholarly-like personality; his demeanor is very tranquil and logical, almost as if he is a professor of the Sanctuary. He is good friends with Taurus Aldebaran, and is very respectful to the 243-year-old master Libra Dohko.
Before the start of the series, Mu removed himself from Sanctuary after suspecting that his mentor (and then-Pope), Aries Shion, had been quietly assassinated. He relocated to Jamir, a mystical land that only those with a high level of either psychic or Cosmo power can enter. From here, Mu severed all ties to Sanctuary and lived a life of solitude with his apprentice, the young psychic Kiki. It wasn't until years later, when he met Dragon Shiryu and Pegasus Seiya, that he began to become involved in the affairs of Sanctuary again.
Although he has left Sanctuary many years ago, Mu was bound by duty to return to Sanctuary when Saori Kido declares that she is Athena, and challenges the Pope for leadership of Sanctuary. When the Bronze Saints were sent to make their run through the 12 Temples, they see that Aries Mu was the guardian of the first temple. Instead of fighting them, Mu tells the Bronze Saints that their Cloth withstood small amounts of damage over the past battles, and asks them to spend 1 hour here so he could repair them. He also reveals to the Bronze Saints about the "Seventh Sense," the strongest cosmo that is mastered by the Gold Saints.
Mu is the man to go to when a Saint needs his or her cloth repaired. Although every individual Cloth has the ability to regenerate itself after a battle, a hands-on restoration from Mu is needed if a cloth has been severely damaged to the point of death.