Who is Erik from If I Die in a Combat Zone, Box Me Up and Ship Me Home and what is their importance?

Erik is a calm and skinny college graduate, who becomes friends with O'Brien during basic training. Both are intellectuals, and they spend hours bonding over literature and analytic reasoning. Both have traveled to Europe and both disagree with the Vietnam War. Unfortunately, they are separated in Vietnam, and although, they don't see each other, they correspond by mail.