Private First Class Koster was a member of the 8th Cavalry Regiment, 1st Cavalry Division. He was taken Prisoner of War while fighting the enemy near Unsan, North Korea on 2 November 1950 and died while a prisoner on 27 February 1951.

Koster was awarded the Combat Infantryman Badge, the Prisoner of War Medal, the Korean Service Medal, the United Nations Service Medal, the National Defense Service Medal, the Korean Presidential Unit Citation and the Republic of Korea War Service Medal.