Aries Mu guards the first zodiac temple within the Sanctuary. Trained by the previous Aries Gold Saint, Shion, who also served as Grand Pope, Mu became a Gold Saint at age seven, recognized as a prodigy. Suspecting Shion had been assassinated and replaced by an imposter, Mu severed ties with the Sanctuary and retreated to the remote Himalayas of Jamir. He lived there in isolation with his apprentice Kiki, mastering the art of cloth repair passed down from Shion. This exile lasted until the Bronze Saints, led by Dragon Shiryu, sought his expertise to revive their damaged cloths. Mu agreed only after Shiryu demonstrated unwavering commitment by willingly offering his blood, a necessary component for the restoration. During the Hades Sanctuary arc, Mu confronted a profound crisis when deceased Gold Saints, including his former mentor Shion, returned as spectral warriors under Hades' command. Shion, wearing a dark Surplice, arrived alongside Gemini Saga, Aquarius Camus, and Capricorn Shura, tasked with assassinating Athena within twelve hours to avoid permanent return to the afterlife. Mu detected conflicting emotions from the resurrected Saints, sensing "tears of blood" that indicated reluctance and possible hidden motives despite their hostile stance. This internal conflict intensified when Cancer Deathmask and Pisces Aphrodite, also resurrected as Specters, arrived. Shion intervened, forbidding Mu from attacking them, creating a tactical and emotional impasse for Mu facing his revered master's direct command. The arrival of Pegasus Seiya complicated the situation. Seiya, defying Athena's orders, attempted to assist Mu against the Specters. Prioritizing Seiya's safety and recognizing his unpreparedness, Mu employed a calculated deception. He used his Crystal Wall technique against Seiya, making it appear he killed the Bronze Saint to convince the Specters of his demise, then teleported Seiya away. Later, after Deathmask and Aphrodite departed, Seiya returned. Emotionally drained yet acknowledging Seiya's stubborn determination, Mu reluctantly allowed him to stay and witness the confrontation. Mu later faced the Specter Papillon Myu. During this battle, he demonstrated strategic intellect and mastery of psychic powers, analyzing Myu's techniques and countering them by creating the Crystal Net—a barrier specifically designed to immobilize Myu's deadly butterflies. Securing this advantage, Mu defeated Myu decisively. The conflict peaked when Virgo Shaka, seeking to understand Saga, Camus, and Shura's true intentions, engaged them within the Virgo temple. The fight escalated, leading Shaka, Saga, Camus, and Shura to unleash the forbidden Athena Exclamation—a technique requiring three Gold Saints to combine their cosmos, resulting in catastrophic power. Mu, alongside Leo Aiolia and Scorpio Milo, intercepted the combatants near the Libra temple. Recognizing the immense danger of the ongoing Athena Exclamation clash and convinced Shaka's sacrifice was necessary to defeat Hades, Mu made a difficult decision. He proposed that he, Aiolia, and Milo counter with their own Athena Exclamation. This choice violated Sanctuary law due to the technique's destructive nature and requirement for multiple Saints to attack a single target, but Mu framed it as a necessary act to honor Shaka's sacrifice and achieve a greater strategic objective against Hades' forces. The collision of the two Athena Exclamations created a massive explosion. The revived Gold Saints ultimately achieved their true goal: delivering a hidden message to Athena, leading her to sacrifice herself to reach the Underworld. Mu's final stand occurred during the assault on Hades' temporary stronghold, Heinstein Castle in Germany. Alongside Aiolia and Milo, he confronted Wyvern Rhadamanthys, one of Hades' three powerful Judges. Despite fighting valiantly, Mu was ultimately defeated and killed by Rhadamanthys within the castle.