Jasper Sitwell was a agent of HYDRA operating within the ranks of SHIELD. He assisted in the encounter in New Mexico, and the tracking of Loki. He was also a good friend to Phil Coulson, until HYDRA revealed themselves. He was later interragated by Captain America, Falcon, and the Black Widow for info on HYDRA's plan of world conquest. After he talked, he was killed by the Winter Soldier.
Agent Sitwell was a fast talking communicator who is "legendary" within S.H.I.E.L.D. for his ability to play the "patsy".