Peggy Carter, also known as Margaret Carter, is the older aunt of Sharon Carter (retconned after her initiail appearances where she was Sharon's older sister) and was a girlfriend of Captain America during World War II. Peggy Carter was an American agent serving with the French Resistance during World War II. Eventually she was ordered to protect a secret laboratory, posing as an old store, but actually housing the Super-Soldier Project. She was disguised as an old shop-keeper and was near the site where Steve Rogers became Captain America after he was injected with the Super-Soldier Serum. She was named Agent 13 during this mission. Her talents as a soldier and a spy could however not prevent a Nazi-spy from entering the secret laboratory. The spy had joined a few high-ranking American army officers to the Super-Soldier Project as his American alter ego Fredrick Clemson. There he killed the Professor responsible for the Serum.
Peggy Carter was created by Joe Simon and Jack Kirby and first appeared in Captain America Comics Issue 1 (1941) where she was known under the name "Agent 13". Years later, it was revealed that "Agent 13" was in truth Peggy Carter, Captain America's love interest during the war. Her first modern-day appearance was in Tales of Suspense 84.
Major Story Arcs
After the shock that brought about the death of the man whom had made the Super-Soldier Serum, Peggy became even more competent a spy and detective and more determent to stop the Nazi's from world wide domination. During the war, Peggy and Captain America started working together and even became romantically involved. The two fought on various different missions during the War and their relationship became stronger and stronger. This however did not last for long, since Captain America seemingly died at the end of the war. Peggy moved on with her life, but could never forget her time with Captain America and it would affect her life for many years to come.
Years later Peggy, now older, would tell her niece Sharon Carter about the war and her adventures with Captain America and the allies. These stories inspired Sharon to follow in her aunt's foodsteps and become an agent of S.H.I.E.L.D.. It was at this time that Peggy learned of the fact that Steve Rogers, her former flame better known as Captain America, was still alive and revived in these modern times. Peggy had however grown into an older woman by that time. Steve and Peggy met on many occasions where she helped out Steve numerous times. Age however began to take it's toll on Peggy and she was hospitalized. More recently she passed away from old age. Captain America and Peggy's niece Sharon morned for the loss of a great woman.
MC 2 Universe
She took her in after her mother Sharon Carter was killed in a bad car accident. Peggy would tell Shannon stories about her time with the Avengers when she was younger, Peggy also got Shannon a job at the Avengers Museum with her connections to the Avengers and she also called in a favor to the Chief-of-Staff of the museum.
In Other Media
Captain America: The First Avenger (2011)
Peggy is played by Hayley Atwell. Peggy is a British officer in the American Army and a member of Strategic Scientific Reserve. During the training of the soldiers, she came across Steve Rogers a frail, weak young man with a passionate heart and a great will to serve his country, which Peggy took an interest in. When Steve was picked for the Super Soldier Project, it had been her responsibility to escort Steve safely to Dr.Abraham Erskine. She had helped in raiding the Hydra Base, during the final battle. When Johann Schmidt was defeated, Steve had to force the plane to land in the North Atlantic Ocean, to prevent the destruction of America, Peggy knowing that this would be the last time to hear his voice, did not give up hope, of there being another option than the one Steve took. Even after his 'death' she did not forget him and was still part of the SSR, which eventually became SHIELD.
The Avengers (2012)
Though she doesn't appear in the actual film, there is a deleted scene on The Avengers Blu-Ray that states that Peggy is still alive in present day. Steve Rogers is reading over the files of his previous squad mates, all of whom are deceased except Peggy. He contemplates calling her, as her home phone number is listed in her file, but decides against it.