Kevin O' Brien first appeared as himself in Iron Man vol 1 #31 and then first appeared as the Guardsman in Iron Man vol 1 #43. The first Guardsman died in Iron Man vol 1 #46.
O' Brien was head of the research department at Stark Industries and a close friend of Tony Stark. O' Brien also knew that Stark was Iron Man. Stark had O' Brien build the Guardsman armor in case of an emergency.
A time came when O' Brien put on the Guardsman armor to help Iron Man in battle but a malfunction in the cybernetic circuitry stimulated jealousy and rage in the brain. The Guardsman behaved in a dangerous and erratic manner. Later, the Guardsman accidentally died when Iron Man attempted to stop his rampage.
Iron Man: Armored Adventures
In episode 12, "Seeing Red", the second Crimson Dynamo appeared. Anton Harkov invented nanorobots, but they did not work out. Obadiah Stane then came to him after Project P.E.G.A.S.U.S. denied funding; the experiment was going to close, and Stane wanted to help in exchange for the Crimson Dynamo armor. Stane's bodyguard, O'Brien, wore the suit, which was repainted in black with red stripes. Later, Iron Man discovered a collapse in the subway, where he met O'Brien. O'Brien won against Iron Man, and took him to P.E.G.A.S.U.S., where Harkov scanned the Iron Man armor. Iron Man escaped with Potts. Some time later, he returned in a new Buster armor. He easily broke into Project P.E.G.A.S.U.S. and loaded the Technovore virus Stane's computers to corrupt the information. Later, fighting the Crimson Dynamo, Iron Man destroyed the armor, and O'Brien fled. O'Brien was voiced by Brian Drummond.