Sharon Carter, alias Agent 13, is a fictional character, a secret agent in the Marvel Comics universe. She is an ex-field agent of S.H.I.E.L.D. under Nick Fury and occasionally the girlfriend of Captain America. In original continuity, Sharon was said to be the younger sister of Peggy Carter, the wartime love interest of Captain America, but she was later retconned as Peggy’s niece because of the unaging nature of comic book characters.
This picture is of the cosplayer “sootydragon” dressed as Agent 13. Picture was taken from Tumblr.