Aside from the Budapest reference and the fact that Hawkeye convinced Widow to defect to S.H.I.E.L.
That one quintessentially Joss Whedon line from The Avengers told fans everything that they needed to know about Black Widow and Hawkeye; they have worked together for years, and they have been through the wringer more times than they can count.
D, we don’t know much.
Out of all the characters in the current Avengers line-up, they are the two whose stories have been waiting in the wings longer than anybody else.