Dr. Streiten is a scientist working with S.H.I.E.L.D.
It is known that Dr. Streiten is at least above clearance level 7, as he knew about Phillip Coulson's (apparent) death, far more than Coulson himself, seemingly.
Agent Coulson hunts him down when the Centipede Group's interrogation and attempts at torture restore his memory of his death and revival. Dr. Streiten is morally repulsed by his own involvement in the processes necessary to bring Coulson back, describing them as "procedures that no good doctor would ever allow". He explains that Director Fury brought him in "during the seventh operation", intimating that the entire sequence had passed some point of no return, and when Coulson recalls that "I wanted to die", his immediate response is, "And we should have let you".
Though not seen except in the introductory recap, Dr. Streiten is an important figure in the tale which unfolds in "T.A.H.I.T.I.", since what he said to Coulson in "The Magical Place" proves to be less than 100% accurate. However, Streiten himself has vanished without trace by the time the team find out about the 'Guest House', possibly somewhere similar to the equally absent Director Fury. With Skye's recovery, and Coulson's accession to the position of Director, presumably finding the doctor has become less urgent, and potentially easier.