Malcolm Reed was a major and commander of the MACO detachment on ISS Enterprise, as well as armory officer. He was very eager to kill suspected traitors, and cited that an "accident" might occur while escorting Captain Maximilian Forrest to the brig. (ENT: "In a Mirror, Darkly") When Captain Forrest regained control of Enterprise, he took revenge on Reed by having him placed in the agony booth for several hours, ironic since Reed himself had helped Dr. Phlox invent the booth.
Although Commander Jonathan Archer respected his abilities as a soldier and officer, he didn't fully trust Reed, noting that he tended to be overly ambitious and insubordinate at times. It was for these reasons that when Archer seized command of Enterprise, he made T'Pol first officer, even though Reed, as tactical officer, was next in line under the ship's chain of command.
When Archer commandeered the USS Defiant, Major Reed again served as tactical officer and commander of the MACO detachment. When it was determined that a Gorn named Slar was responsible for key engineering components being stolen and for the violent death of one of the engineers, both Archer and Reed led a team of MACOs handpicked by Reed to kill Slar. Like his counterpart in the normal universe, Major Reed tended to show a suicidal sense of self-sacrifice. This was demonstrated when Reed was severely injured by Slar during the hunt. Reed said apologetically to Archer: "I've failed you captain."
Dr. Phlox later commented to T'Pol that Reed only had a 50/50 chance of surviving his injuries. Phlox also noted that there would probably be several "discreet celebrations" among the crew should he die, suggesting that he was not at all popular among the crew.