Jasper Sitwell is an agent of S.H.I.E.L.D.
Biography
Come the Conqueror
He appeared in "Come the Conqueror" when Kang's forces invaded Earth, monitoring the Helicarrier's weapons and shield systems when they malfunctioned.
Skrull Invasion
He was replaced by a Skrull agent during the Secret Invasion, but did not appear in Prisoner of War. It was most likely he was killed when he was replaced.