Peggy Carter (Hayley Atwell)
Ostensibly the main love interest of Captain America, Peggy Carter also served as one of the first agents of S.H.I.E.L.D. and, thanks to her ABC series Agent Carter, fans have continued to follow the character's story long after Cap went into the ice. Initially introduced as a member of the Strategic Scientific Reserve, Carter has been a key component in bridging the gap between the films and Marvel Studios' TV projects. She has continued to help expand the MCU with brief appearances in both Avengers: Age of Ultron and Ant-Man last year, and the second season of her series will no doubt present this unstoppable hero another case to solve.