Jasper Sitwell became a S.H.I.E.L.D. Agent after having been the top student of his class at the S.H.I.E.L.D. Academy. He became Nick Fury's assistant, at one point even filling in for him as temporary Director of S.H.I.E.L.D.. Later, he acted as S.H.I.E.L.D. liaison to Stark Industries.
Sitwell was specifically assigned to provide for Tony Stark's safety. During this assignment, he fell in love with Whitney Frost who turned out to be the leader of the Maggia. Stark and Sitwell's relationship became somewhat strained when Stark also became involved in a romantic relationship with Whitney Frost.
Sitwell was later critically injured while protecting Stark and was out of action for some time. Despite their conflict of personal interest, Sitwell was again assigned to protect Stark ironically around the same time Stark renewed his romance with Frost. Sitwell eventually sought revenge on Stark for "stealing" the woman he loved, but ultimately fulfilled his obligations as a noble agent of S.H.I.E.L.D..
Jasper Sitwell was an agent level 5 at the interrogation of Daisy Johnson (at 4/5/2001), at the interrogation of Hogbolin (the 4/5/2003) six hours after the one-year-anniversary/retaliation of the Secret War at another interrogation of Daisy Johnson, and at the interrogation of Scorcher two weeks later (the 6/22/06), and was later an agent Level 10 some time after the M-Day.
He was also stated to be an agent Level 8 during the interrogation of Grizzly the 1/7/2003, although this might be an editorial error.