Major McCain was a member of the United States Colonial Marine Corps. He was stationed aboard the USS Verloc, and commanded the mission to combat a Xenomorph outbreak at the Weyland-Yutani research facility on LV-1201.
Not much is known about him, but he was known to be the Marines' leader during the LV-1201 mission. He often made use of Harrison to do several tasks should ever he is unable to deal with situations, such as being unable to spare manpower. It appears he can be tricked easily, proven where he thinks Harrison is dead. Unbeknown to him, he was tricked by the Iron Bears. He and the surviving Marines ends up falling into a trap set by the Iron Bears - which results with them being ambushed by Aliens, before they meet up with Harrison and escape the planet.