Tim O'Brien
This is an autobiographical memoir of O'Brien's, so he is both narrator and protagonist. He is the primary character who interacts with but does not stay anchored with any of the other characters. Although he was raised in a patriotic family in small-town America, O'Brien decided for himself in high school that patriotism does not mean a person should engage in warfare. He became convicted that the war in Vietnam was a farce and a mistake. Despite multiple opportunities to escape the draft, however, O'Brien does go to Vietnam and witnesses the war firsthand. He feels afterward that he was both privileged and manipulated. He survives, giving him both a sense of guilt and of gratitude. Upon returning home he concludes that he was used as a pawn in a bureaucratic nightmare, enacting some kind of alternative justice with which he had no business.
Erik
At basic training, O'Brien makes friends with a fellow soldier named Erik. They share a mutual disdain for the war because of ethical convictions. Because of their anti-war sentiments, both Erik and O'Brien are mistreated by their S.O., making them even more reliant upon one another for support.
Sergeant Blyton
Blyton is the drill sergeant supervising O'Brien during basic training. He's a brash guy who punishes O'Brien and Erik for opposing the war. He thinks they're scared and that's why they won't participate.
Captain Johansen
He is O'Brien's commanding officer once he arrives in Vietnam. He's a competent leader who instills fear in his men in order to prepare them for actual battle.
Major Callicles
Major Callicles offers O'Brien a job as a typist outside of the combat zone. He is charged with investigating the My Lai Massacre, which challenges his personal investment in military honor. He uncovers a real scandal which destroys his confidence in the justice of the war.