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.