Jasper Sitwell is an espionage agent working for S.H.I.E.L.D., 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.
He was sent to New Mexico after Agent Coulson found Mjölnir in the desert. He was among the S.H.I.E.L.D. agents who confiscated Professor Selvig and Jane Foster's research on wormholes. Sitwell became 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 was finally apprehended, Sitwell witnessed Coulson's interrogation of him.
When another wormhole was opened in the desert the next day, 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 was something of Stark's, but Coulson responded that Stark never tells him anything. Immediately after, The Destroyer started a massive attack against the agents, and Sitwell was among the survivors.
Agent Sitwell summoned Agent Coulson to a meeting in a cafe to discuss a major problem: the World Security Council wants Emil Blonsky freed from custody and integrated in The Avengers Initiative. The two of them, knowing the truth about The Abomination, think of a plan to avoid Blonsky's liberation, and decide to send The Consultant (Tony Stark) to annoy General Ross, who has Blonsky in custody, to the point that he'll never release his prisoner.
Sitwell was still serving under S.H.I.E.L.D. during the events of The Avengers, and is seen as a computer expert on S.H.I.E.L.D Helicarrier. He was the one who located Loki in Stuttgart, and during Clint Barton's attack on the ship, Sitwell's abilities came in handy in regaining the control of the Helicarrier despite the heavy damages.
Agent Sitwell is sent by Alexander Pierce on a S.H.I.E.L.D. ship to retrieve Arnim Zola's code from the computers onboard, but the ship is assaulted by Georges Batroc and his pirates, and Sitwell is taken prisoner. Before the negotiations are done, Sitwell and the others are saved by a S.T.R.I.K.E. team led by Captain America, and the data are retrieved by Black Widow. Sitwell comes back to Triskelion, and is assigned to the security of Senator Stern. Both Sitwell and Stern are revealed to be undercover HYDRA agents, and Sitwell takes active part to the hunt of Captain America and his allies after Nick Fury's apparent death. When in company of Stern, however, Sitwell gets captured by Rogers, Romanoff and Wilson, looking for answers. Under threat, Sitwell reveals to them that the new Helicarriers are programmed to kill individuals considered actual or potential threats by HYDRA, such as Bruce Banner, President Ellis or Stephen Strange. When Cap and the others try to take him back to Triskelion to access further informations, however, Sitwell is attacked and killed by The Winter Soldier, who silences him throwing him under a passing truck.
