Jasper Sitwell was an espionage agent working for S.H.I.E.L.D., but secretly he was a double agent for Hydra.
Fury's Big Week
Jasper Sitwell is assigned to the surveilance of Bruce Banner. Even if the World Security Council orders to stop his mission, Nick Fury orders him to keep an eye on the scientist. When Phil Coulson discovers a mysterious alien device landed in New Mexico, Sitwell is re-assigned and leaves Banner's surveilance to Natasha Romanoff.
Sitwell is head of the lab studying the hammer-like artifact found in the crater who also witnesses the intrusion of "Donald Blake" into the base. When the man is finally apprehended, Sitwell witnesses Coulson's interrogation of him.
When another wormhole is opened in the desert, Sitwell goes to the desert to check with Coulson and some other agents, only to witness the arrival of The Destroyer. He asks Coulson if the robot is something of Stark's, but Coulson responds that Stark never tells him anything. Immediately after, The Destroyer starts a massive attack against the agents, and Sitwell is among the survivors.
Jasper Sitwell keeps serving under S.H.I.E.L.D., and is seen as a computer expert on S.H.I.E.L.D Helicarrier. He is the one who locates Loki in Stuttgart, and during Clint Barton's attack on the ship, Sitwell's abilities come in handy in regaining the control of the Helicarrier despite the heavy damages.
Jasper Sitwell is a highly trained professional agent, dutiful and skilled as a high-level agent should be. He seems to be an expert in scientific issues, and often works together with Phil Coulson.