The niece of World War II super spy Peggy Carter, Sharon worked hard to become a S.H.I.E.L.D. agent, earning the designation Agent 13. Soon after Captain America came out of suspended animation, the two met, worked well together and began a relationship. That came to an end for quite a while when Rogers thought Carter died in the line of duty. But in reality, she'd be sent on a secret mission by Nick Fury that went sideways and kept her out of commission for years.
Reunited way down the line under unusual circumstances involving the Red Skull and a Cosmic Cube, the duo eventually got back to business as usual, taking on threats of all sorts. They found themselves separated once again when Carter became a pawn in Red Skull's plan to kill Captain America. After breaking free of that control, she worked alongside the new Captain America (Bucky Barnes), Falcon, and other allies to bring Steve Rogers back!
Since then, things have gotten even crazier for Sharon Carter! In addition to serving with the SECRET AVENGERS (2014), she went into Arnim Zola's Dimension Z to find Steve, who had spent over a decade there, relatively speaking. She managed to line up his escape, but appeared to die in the process. In reality, she spent even more time looking after Ian, Zola's son who Rogers had freed during his time there and raised as his own son. That is, until one of Zola's raiding parties made off with her.