Who is Mr. A. R. Black from Extremely Loud and Incredibly Close and what is their importance?

Oskar Schell's upstairs neighbor, Mr. Black claims to be a 103-year old descendant of Mayflower immigrants, and more convincingly is a half-blinded, deaf, widowed journalist who has covered both world wars and conflicts around the world. He retires when his late wife begs him to spend more time with her. He does and cuts down a tree over which she trips during their courtship and builds a bed from the pieces.