Sharon Carter was the daughter of Harrison and Amanda Carter, two wealthy Virginians, and the younger sister of Margaret "Peggy" Carter, a heroic freedom fighter during World War II who had shared many adventures with Captain America before his supposed death at the end of the war. When Peggy returned home in need of hospitalization from traumatic shellshock, she told her younger sister many stories about Captain America. Inspired by these tales and her own sister'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 sister 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.
Captain America continued to cross paths with Carter as the two adventured, only knowing her as Agent 13. On one adventure, Captain America rescued her from the subversive group AIM, and the two fell deeply in love with each other. After a later assignment bringing down the Nazi Red Skull's "Fourth Sleeper" robot together, Carter was allowed by S.H.I.E.L.D. to reveal her secret identity to Captain America. The two became lovers and remained allies for many subsequent missions.
Carter briefly joined S.H.I.E.L.D.'s so-called Femme Force, an all-women squad of commandos.
Later, Captain America learned that Carter's sister was the woman he had loved during World War II and was able to rescue her from the clutches of the evil Doctor Faustus. It was on a mission against Doctor Faustus that Carter participated in what would come to be believed as her final mission.
Carter was assigned to be the S.H.I.E.L.D. liaison to the New York City Police department while both organization investigated the subversive group called the National Force, which was secretly controlled by Doctor Faustus. Carter fell under the thrall of Faustus' mind-influencing gas during a rally in Central Park, and she was abducted with the others by the National Force's Grand Director, who had also been the fourth Captain America. Carter and others in the group were coerced to march on New York City's Harlem area to burn it down. When confronted by the National Guard, the mesmerized Carter (in accordance with the orders given by Faustus) activated a self-destruct device in her costume and was believed incinerated with other members of the rally.
It was later revealed that Carter's death was staged by S.H.I.E.L.D. director Nick Fury, so that Carter could go on to perform an even more dangerous assignment. However, something went wrong and Carter was believed dead by S.H.I.E.L.D. and left abandoned behind enemy lines. She ended up joining with S.H.I.E.L.D.'s enemies out of resentment toward Fury, and even went on to become an agent of the Red Skull. However, when Captain America defeated the Skull's plans the two were reunited and Carter returned home.
Still wishing to confront Nick Fury, Carter learned not only that Fury was believed dead (killed by the vigilante known as the Punisher) but that his death and funeral were staged. Carter surreptitiously infiltrated the agency to investigate only to find herself pursued by S.H.I.E.L.D. and wanted for treason. To avoid capture, Carter was forced to escape through an energy portal she found in an abandoned S.H.I.E.L.D. safehouse. There, she found herself in Nazi territory during World War II, where she encountered Nick Fury.
Fury had found documents which questioned the fate of his predecessor, the S.H.I.E.L.D. director code-named Fallen Angel, whom he had believed was killed by agents of HYDRA. In fact, he was abandoned by S.H.I.E.L.D. after HYDRA kidnapped him. Fury set about tracking the Fallen Angel, but as the situation was politically volatile, Fury diverted attention from his investigation by staging his own death. His search led him to the "Backslide," a supposedly abandoned S.H.I.E.L.D. project which created a portal from a fabricated Cosmic Cube. Ultimately, the Fallen Angel hoped Fury might use the portal to go back in time, prevent S.H.I.E.L.D. from being formed, and thus gain revenge for his abandonment. Fury disagreed and he and the Fallen Angel struggled. They both ended up traveling through the portal, which did not lead back into time, it merely created a reality based on the memories of war from the two soldiers. Fallen Angel took his own life and Fury was left, trapped, fighting a stalemate war for months.
Carter's presence allowed the Backslide's reality to change, offering a way out. Together, Fury and Carter battled their way free, as on the other side of the portal S.H.I.E.L.D. agents were about to destroy the portal, since Fury was the only one who could open it. Carter and Fury managed to escape moments before the portal's destruction, although Fury disappeared in the explosion.
Soon afterward, Carter re-enlisted with S.H.I.E.L.D.. Sharon served a brief term as Executive Director of S.H.I.E.L.D. during Nick Fury's absence. She is currently a field agent once more, reporting directly to the new Executive Director as a liaison officer specifically assigned to support and report on Captain America's activities. Captain America and Carter continued to work together and remained close. They began investigating the Winter Soldier.
Following the conclusion of the Civil War, Agent 13 found herself a pawn of Doctor Faustus and the Red Skull. Using Faustus' expertise in hypnosis, the villains activated a subconscious command in Sharon's mind, forcing her to shoot and kill Captain America. 
Sharon eventually discovered that she was pregnant.  She tried hiding the pregnancy from the Skrull but he eventually found out and planned on using her child for his own purposes.
Though on the bad guys' side, Sharon managed to release Steve's partner, Bucky, aka the White Soldier, to Black Widow and Falcon.
Sharon soon found what she believed to be Steve, alive. However, upon closer look, she realized it was the 50s Captain America, the Grand Director. Refusing to let such an abomination live, Sharon attempted to kill him only for Aleksander Lukin, whom the Red Skull was inhabiting, to shocked her from behind. She was then taken to a med ward. Eventually, she was able to escape and take the Skull's daughter, Sin, hostage. However, a fight between them resulted in Sharon getting stabbed in the stomach and losing her baby. Dr. Faustus helped her, reactivating her tracking device and made her forget being pregnant. When SHIELD assaulted the compound, Arnim Zola took Sharon to a device that the Skull acquired from Doctor Doom. Sharon managed to fight against the procedure and managed to kill Lukin. She was rescued by Falcon.
Agent 13 possesses the normal human strength for a woman her age, height and build who engages in regular intensive exercise.
Agent 13 has access to a wide variety of conventional and high-tech S.H.I.E.L.D. weaponry.