Why doesn't Phineas have a last name in A Separate Peace?

Phineas's lack of a last name is intentionally done by the author, John Knowles, to emphasize the intimacy of the friendship between him and Gene. By not giving Phineas a last name, Knowles suggests that Phineas is more than just a character, but a symbol of innocence, purity, and the nostalgia of youth.