Who is Oskar Schell from Extremely Loud and Incredibly Close and what is their importance?

The novel's principal narrator and protagonist, Oskar is a precocious, psychologically troubled nine-year-old, who occasionally claims he is older or younger to get sympathy or respect. His business card describes him comprehensively as an "inventor, jewelry designer, jewelry fabricator, amateur entomologist, Francophile, vegan, origamist, pacifist, percussionist, amateur astronomer, computer consultant, amateur archeologist," and collector of various things. He keeps a thick scrapbook entitled Stuff That Happened to Me. His favorite book, astrophysicist Stephen Hawking's A Brief History of Time, is far beyond his reading level.