Major Greg Duprey is a former U.S. cavalryman who had a change of heart after meeting the Sioux. Accompanying Dr. David Yellin, he got stuck on Mars. After his arrival in the ill-fated first capsule, Duprey, together with Dr. David Yellin and Richard Sherman went out to explore the mines. It turned bad when he and Sherman got stuck in the mines near Syrtis Major at 10°N 71°E after a cave-in while searching for iron ore for Andrew Carnegie. After the Avatar had freed him in Martian Dreams, he gladly vouched for entry to Olympus. At the end, he returned to Earth.