Jonathan F. "John" Walker is a former Captain of the United States Army's 75th Rangers Regiment who was chosen to succeed Steve Rogers as the second Captain America. However, his aspirations of living up to Rogers' legacy attracted the ire of Sam Wilson and Bucky Barnes in their pursuit of the Flag Smashers, during which he took the Super Soldier Serum after failing to apprehend Karli Morgenthau. When his partner and best friend Lemar Hoskins was killed in action, Walker brutally murdered a surrendering foreign national, leading to him being publicly stripped of his mantle and discharged with total loss of rank. Walker confronted Morgenthau again in New York City, before abandoning his revenge to join Wilson and Barnes in thwarting her attack on the GRC. Despite his discharge, Walker was recruited by government agent Valentina Allegra de Fontaine to continue serving America in secret, rebranding him as U.S. Agent.