Copperhead was a person who lived in the Capital Wasteland.
Copperhead sent Three Dog a weapons cache note, with coordinates to the military weapons cache uncovered in Hamilton's Hideaway. Copperhead's dialogue makes the Good Fight sound like an actual organization. This makes Three Dog's banter less of a motivational speech and more of a recruitment campaign.
Copperhead is a member of the Good Fight, and personally knows Three Dog.
Copperhead is mentioned only in Fallout 3.