Marcus Bretner is a former N7 Alliance Marine infantrymen and a Grizzly, Mako, and Hammerhead tanker before his promotion to an officer rank. Shortly before his disappearance, he was serving as a Major in the Alliance Military. Marcus is a no non-sense marine, but is particularly charismatic. Despite his men having called him a "hard-ass", Marcus cares deeply about his fellow soldiers, especially those under his direct command.
Marcus has seen years of combat, since before the First Contact War, and many years after. Even with his older age, Major Bretner keeps up with the best of his soldiers, and is just as strong and agile as ever; Possibly due to unknown cybernetics, augmentations, or cryogenic sleep, and perhaps a combination of the three.
Almost a year before he disapeared into the Terminus Systems, Marcus had ties with Cerberus. Although not fully converted to a "human above all else" agenda, he was interested in advancing the human race significantly. Marcus became a top lieutenant to the Illusive Man, having known him before the founding of Cerberus. Eventually, though, his morality got the best of him, and he left alongside a Cerberus assassin and friend, Terren Hagard. Despite a mutual respect, the Illusive Man set out to kill his former ally.