Who is Colonel James from Swan Song and what is their importance?

Colonel Macklin is a Vietnam war veteran who was a POW for a time. He was kept down in a hole with two other soldiers who he killed, as per the Shadow Soldier's instruction, so he could have more food to himself. He is paid to endorse Earth House as a secure state-of-the-art fallout shelter, though he knows the place has structural problems and it ultimately falls to pieces during the bombings. Roland cuts his hand free for him when it is still in rock in the bowels of Earth House, and they eventually take over a camp on the Great Salt Lake and form the Army of Excellence. His Shadow Soldier spurs him to do everything difficult, echoing his mantra of discipline and control in his ear.