Private Caston Gage was a marine in the Dominion Marine Corps.
Biography
Gage survived a zerg attack on Mar Sara which killed his parents. He joined the Corps; Private Gage was assigned to Meatbag Squadron, a recruit unit. Meatbag Squadron deployed for training exercises at the abandoned Confederate Ghost Academy on Tarsonis.
The Academy was occupied by the Scantid Pirates, and the marines eliminated them without loss. Gage was discomfited by the visceral remains. He recovered his nerve by killing a feral purple-eyed overlord with a BOSUN FN92 rifle. The companion green-eyed overlord evaded his shots. Unknown to Gage, these overlords were among the original Gargantis proximae assimilated into the Zerg Swarm and had regained their individuality.
"Green Eyes" resolved to punish the marines for the death of "Purple Eyes". Gage realized something was afoot by the way "Green Eyes" observed the marines, and by the unexpected appearance of controlled zerg near the Academy. He could not convince his comrades of the danger before the zerg attacked.
"Green Eyes" ordered the zerg to kill all the marines except Gage. The two met alone outside the Academy. The overlord wanted the marine to kill it. Only then did Gage realize what he had done; the overlords had resumed their relationship once freed from the hivemind, and Gage had destroyed that reunion. Disgusted with himself, Gage could not bring himself to kill "Green Eyes". The overlord departed to mourn.
Gage deserted to avoid rescue and the neural resocialization that would cause him to forget. He buried the other marines and abandoned his armor before wandering off into Tarsonis City.
References
Gavin Jurgens-Fyhrie. "Just an Overlord." (March 21, 2013). Blizzard Entertainment. StarCraft Lore: Just an Overlord Accessed 2013-03-25.