Who is Dr. Fein from Extremely Loud and Incredibly Close and what is their importance?

Oskar Schell's psychiatrist whom Oskar sees only because his allowance depends on it, Dr. Fein annoys his young patient with condescension and trying to be a buddy. Oskar amusingly twists a word association game until it makes him self-conscious. Using a stethoscope, Oskar hears Fein through the door mention "hospitalization," and panics. During the performance of Hamlet, as he pictures beating Jimmy Snyder to a bloody pulp, Oskar also pictures doing it to Fein, and yearns to tell the doctor "fuck you," but settles for a Dad-like shrug of his shoulders.