O'Brien found himself seized with sudden attraction for Stark's girlfriend Marianne Rogers, and became extremely jealous of Stark's power, looks and fortune. At the same time, Simon Gilbert, then Chairman of the Board of Stark Industries' stockholders, grew alarmed that Stark was moving out of munitions production and mapped strategies with the board to seize controlling interest in the firm from its principal stockholder, Stark himself. O'Brien—clad in armor and calling himself the Guardsman—offered to aid the board in their plot against Stark. As a show of support, he agreed to quell a protest rally outside the plant. The Guardsman aimed his repulsor rays at the crowd, injuring four protestors. Sickened by what he did, O'Brien turned on the renegade board members, roughing them up. But seeing Tony Stark with Marianne Rogers caused him to once again become unbalanced and he went outside to vent his rage on the growing crowd of protestors. Stark donned his Iron Man armor and engaged the Guardsman in battle to prevent him from doing more damage. Losing, the Guardsman sought refuge in an experimental tank. In an attempt to stop him without hurting him, Iron Man trained his repulsor rays on the vehicle and accidentally hit its fuel supply. The tank exploded, killing O'Brien.
O'Brien finally hired an unscrupulous private investigator to get him inside the Long Island plant. There O'Brien located the Guardsman armor and put it on, determined to use it to bring Iron Man to justice. However, the malfunction in the cybernetic circuitry that affected his brother also affected Michael's brain and he flew into a rage seeking to kill Iron Man. Iron Man confronted his attack and finally convinced him that the armor was causing him to act insanely. O'Brien collapsed on rebelling against his own urge to kill Iron Man. Stark took O'Brien into custody rather than press criminal charges, hoping to convince the man of his innocence in Kevin's death. While Stark worked on a new set of armor, the Japanese mutant Sunfire attacked the plant. Stark was unable to put on his new armor since it had not yet cooled, and unable to find any of his spare suits of armor since a saboteur had stolen them. Hence he was forced to put on the Guardsman armor to fight off Sunfire. O'Brien witnessed Stark's heroic attempts to save lives over a video monitor, aware that Stark was risking a major heart attack by the strenuous activity. Escaping confinement, O'Brien decided he was wrong about Stark and determined to help him by donning the now-cooled Iron Man armor. Thus clad, he was mistaken for the real Iron Man and kidnapped by the Mandarin. Stark discovered an old set of Iron Man armor that the saboteur had overlooked and went to China to rescue O'Brien. Upon rescuing him, Stark chose to reveal his true identity to O'Brien before he went off to battle the Mandarin. O'Brien flew back to New York where he took custody of the Guardsman armor once more.
After Iron Man defeated the Mandarin, Stark fixed the malfunction in the Guardsman armor's circuitry so it was safe to wear. Michael O'Brien was determined to use the armor to vindicate both his brother's and his own senseless actions. When Stark International was taken over by the criminal Midas, the Guardsman joined with a number of Stark's other friends and allies to battle Midas' men. He was turned to gold by Midas' power, but was later restored to flesh. Having quit the police force, O'Brien interviewed for the position of security director at Project: Pegasus, the government energy research facility, after its previous director, Quasar, quit. With a high recommendation by Tony Stark, O'Brien got the job. Sometime later, while guarding the Project, O'Brien's Guardsman armor was damaged in a battle with the subhuman Lava Men. It was later repaired at the Project's expense. O'Brien, as the Guardsman, has been living and working full-time in the Project since his appointment and has performed his duties capably.
Later, he was the security chief at Avengers Mansion for some time.
Each palm of the Guardsman armor's gauntlets contains a charged ion "repulsor ray" emitter capable of 45 seconds continuous discharge at a range of 40 feet before irregularities in plasma cone diffuse the beam.
Kevin O'Brien had earned a Ph. D. in engineering, with expertise in engineering and was a gifted inventor. He also used a "stun-ray" device as a weapon.
Michael O'Brien is a good hand-to-hand combatant, and was coached by Captain America. As a policeman, he carried a police issue handgun, and as Avengers security chief he used "stun guns" and other advanced weaponry.
Both Kevin and Michael suffered from emotional instability that was exacerbated by the cybernetic circuitry in the original Guardsman battle-suit, causing both men to go insane. Michael O'Brien was able to wear the Guardsman armor safely after psychologically coming to terms with his trauma over his brother's death. The armor circuitry has since been modified by Anthony Stark to prevent its causing such adverse effects.