Mu is a veteran mobile armor ace, and a longtime rival of ZAFT officer Rau Le Creuset, with whom he demonstrates a strange extrasensory connection. He is famous in many parts of the Cosmic Era universe as the "Hawk of Endymion" for destroying five ZGMF-1017 GINN mobile suits with his TS-MA2mod.00 Moebius Zero mobile armor at the Battle of Endymion Crater, a feat previously considered impossible. The Battle of Endymion Crater also leaves him as the only remaining OMNI pilot capable of handling the Moebius Zero, the wire-guided gunbarrels of which require a high degree of spatial awareness not commonly possessed by Naturals. A laid-back man with a thick-skinned attitude toward war, Mu is casually boastful of his preternatural abilities, sometimes calling himself "the man who can make the impossible possible."
While on the Archangel, Mu advises Gundam SEEDs young protagonist, Kira Yamato, on the finer points of both mobile suit combat and coping with the stress of a combatant's life. He also provides the benefit of his greater experience to Murrue, contributing a great deal of the strategies employed by the Archangel early in the series, and serving as a pragmatic midpoint between Murrue's idealism and combat commander Natarle Badgiruel's strict reliance on military regulations.
Upon the Archangel's arrival at JOSH-A, Mu is transferred off of the ship and ostensibly assigned a position as a flight instructor in California. However, at the last minute he chooses not to board his transport and turns back, possibly because of his growing romantic rapport with Murrue. As a result, he is the first to discover both ZAFT's impending attack and the Cyclops system installed beneath the base; his return to the Archangel to warn its crew, along with Kira's arrival in the ZGMF-X10A Freedom, enables the ship to survive the battle and the detonation of the base.
He remains on board the Archangel following its defection to Orb, discontinuing the use of his FX-550 Skygrasper in favour of the rebuilt GAT-X105 Strike shortly before the Battle of Orb, and beginning an openly romantic relationship with Murrue. Later, during the Battle of Mendel, he and Kira confront Rau Le Creuset at the abandoned Mendel colony and learn of Rau's origins as a clone of Mu's father Al Da Flaga, explaining their extrasensory awareness of one another and the root of Rau's plans to goad humanity into wiping itself out.
Mu is presumed killed at Jachin Due when the Strike is destroyed protecting the Archangel's main bridge from the Dominion's "Lohengrin" positron cannon. However, in the Gundam SEED: Special Edition compilation of the series, the footage of the destroyed Strike was edited so that Mu's helmet no longer appeared amidst the wreckage, retconning his death from definite to assumed in order to allow for his return in sequel series Mobile Suit Gundam SEED Destiny.
Although a ruthless and cunning leader, Neo commands a great deal of trust from his subordinates; he demonstrates some sympathy for the Extended pilots, who are viewed as equipment by most of the Earth Alliance Forces. However, he also manipulates them over the course of the series, and repeatedly breaks promises made to or involving them at orders from Djibril. He allows Stella's memories of being rescued by Shinn Asuka to be erased, after promising her he would not; later in the series he assigns Stella as the pilot of the GFAS-X1 Destroy after promising Shinn that he would not send her into battle again, and plays on her fears to encourage her to fight, an action which leads to Stella's death at the Battle of Berlin.
The crew of the Archangel determine that Neo's physical data is a 100% match to their records of Mu. Neo initially denies being Mu or knowing anything about him; however, over the course of the next ten episodes there are several instances in which he demonstrates a degree of unconscious familiarity with the ship, as well as having a knee-jerk emotional reaction to Murrue's presence, resulting in growing confusion about his identity. Ultimately, after Murrue releases him from the Archangel and provides him with a FX-550 Skygrasper for transportation, Neo chooses to return to support the Archangel against the Minerva during the Second Battle of Orb, in the course of which he repeats Mu's assertion that he can "make the impossible possible" and has a brief flashback of the emergency landing Mu made on the Archangel during the Battle of JOSH-A. Following the battle, Neo confides in Murrue his doubts about his identity, confessing that he returned because something in him knows her, and asking to remain on board the Archangel. Murrue acquiesces, and Neo is thereafter commissioned as a captain in Orb's military.
Neo takes over the ORB-01 Akatsuki when the Archangel returns to space and docks at Copernicus City. He goes to Kira, Athrun, Lacus, and Meyrin's rescue when they run into an attempted assassination on Lacus on a recon mission. However, as he arrives in the Akatsuki, Meer Campbell, is shot by her own bodyguard. Neo later regrets not getting there in time. During the attack on Station One, he uses the Akatsuki to protect the Archangel from the Minerva's Tannhäuser positron cannon in a mirror of the events of the Second Battle of Jachin Due; this time, the Akatsuki's reflective armor enables the mobile suit to survive the blast, and in the process Neo regains his memories as Mu. Following the destruction of Station One, the recovered Mu joins Athrun Zala in destroying the Requiem before it can be fired on Orb.
At the end of Gundam SEED Destiny: Special Edition, Mu and Murrue are seen together at Orb, watching the sunset.
Arguably, it is not until Mu pilots the ORB-01 Akatsuki that the depth of his skills are seen, as it is effectively the first time which Mu pilots a unit on-par with the most advanced mobile suits of that time. Although the only time he enters combat is during the attack on Station One and during The Battle of the Messiah, the Akatsuki comes away from each of the fights unscratched, having both times destroyed an impressive amount of enemy mobile suits, as well as defending the Archangel and the Eternal from the Minerva.
In Super Robot Wars W, Mu is one of the few SEED characters to appear in the first half of the game. He first appears as an enemy, part of the military force sent to stop the Nadesico from leaving Earth, but after the force is driven off, he returns to help them against a Jovian strike force as a playable character. He appears again as part of a force attempting to repel Radam forces on the Orbital Ring, where he is again playable.