Agent Coulson is an excellent ally. He has the key statistics you would want from a cost 3 ally (thwart 2 and hit points 3), but he effectively has a net cost of 2.
Agent Coulson looks on the surface like he is a very specialized ally you would only want to take in a heavy preparation deck (i.e. Black Widow), but in fact this isn't the case, he can be used pretty easily by just about anyone. All you need is a single preparation card in your deck that you don't plan to leave in play (perhaps a Counterintelligence that you use at the first opportunity), and he will be able to find and draw that card from the deck or discard pile to get the benefit from his ability. If the card is in your hand instead, just spend it to play Agent Coulson, or for some other purpose, so that it goes to your discard pile and you can pull it back out.