Jasper Sitwell is an American who works for S.H.I.E.L.D.'s Internal Affairs division.
After mustering out from the US Army Special Forces, Sitwell served seven years in Cryptanalysis in the CIA before Fury personally picked him to head up the Internal Affairs division of S.H.I.E.L.D. Nobody at S.H.I.E.L.D. likes Jasper. They see him as a glorified accountant, no matter how many Purple Hearts he won in Iraq (it was, for the record, three). He’s Internal Affairs, so it’s his job to make sure the monsterous machine of an agency they’ve got is running smoothly. That means sticking his nose where other agents don’t think it belongs and asking the questions nobody wants answered. He doesn’t owe any Section Director or Station Chief his job so he doesn’t have to compromise his judgments for political expediency or personal likability. In other words, he’s exactly the sort of person every successful organization needs. Nobody at S.H.I.E.L.D. likes Jasper but Nick Fury. But Jasper knows Nick Fury's opinion is the one that matters.