Sharon Carter was the daughter of Harrison and Amanda Carter, two wealthy Virginians, and the niece of Margaret "Peggy" Carter, a heroic freedom fighter during World War II who had shared many adventures with the great hero Captain America before his supposed death at the end of the war. When Peggy returned home in need of hospitalization from traumatic shell-shock, she told her niece many stories about Captain America. Inspired by these tales and her own aunt's exploits, Sharon decided to become an agent of S.H.I.E.L.D., the international espionage agency.
On her first major assignment, Sharon Carter, code-named Agent 13, was chosen to pick up a cylinder containing the powerful explosive called Inferno-42 from a S.H.I.E.L.D. agent who had stolen it from THEM, the governing body of the subversive organization HYDRA. By coincidence, Captain America, in his secret identity of Steve Rogers, saw Carter on her way to receive the cylinder and was struck by her close resemblance to her aunt Peggy. However, Rogers had no idea who Sharon Carter was, having known Peggy only by her cover identity.
Soon afterwards, Sharon Carter was attacked by the costumed mercenary Batroc, who had been hired by THEM to recover the cylinder. As Captain America, Rogers came to her assistance and, while he and Batroc battled each other, Carter made off with the cylinder, unaware that its casing had been damaged, activating the explosive within. Learning from Batroc that she was a S.H.I.E.L.D. agent, Captain America went in pursuit of her, and succeeded in saving her life and preventing THEM from making use of the explosive.