A dependable agent in the background of several films, who turns out to have been working for the other side the whole time.
Jasper Sitwell is a S.H.I.E.L.D. agent who spends the first years of the shared story in the margins: at the hammer site in Thor, across the table from Phil Coulson in two of the short films, briefly in The Avengers. Maximiliano Hernández plays him as the steady, slightly harried colleague you never think about twice, which is the whole trick.
S.H.I.E.L.D. is the spy agency that runs through the early films, and Hydra is the organisation that grew inside it in secret. Sitwell was one of Hydra’s sleeper agents. Captain America: The Winter Soldier is where that comes out: Steve Rogers, Natasha Romanoff and Sam Wilson catch him, force Hydra’s plans out of him, and he is killed for that betrayal.
Agents of S.H.I.E.L.D., the television series, fills in the earlier part of the story. Adam Faison plays a younger Sitwell in a flashback, and the fifth season episode “Rise and Shine” reveals that he attended a Hydra academy in his youth. So he was never a S.H.I.E.L.D. agent who turned. He was Hydra before he joined.
Minor spoiler
The Winter Soldier kills him in an ambush, and it is in that ambush that Rogers recognises the Winter Soldier as Bucky Barnes. Avengers: Endgame brings Sitwell back only because the heroes travel to 2012: Wikipedia counts the Sitwell they meet there as a variant from an alternate timeline, and this entry does too.