Howard W. Campbell, Jr., like Bertram Copeland Rumfoord, actually appeared in a previous Vonnegut novel, called Mother Night (1961). Like Roland Weary, Campbell has this odd, invented identity for himself, a bizarre mix—or maybe the most bizarre mix—of Nazi and American Cowboy.
He is a traitor to the U.S. who has joined the German army and written essays about American soldiers and their behavior in German captivity. We also find out that he hangs himself at the end of the war. Campbell believes that American soldiers make disgusting prisoners because they have no self-respect: they come from a country that hates the poor, and, because they themselves are often poor, they hate themselves.
While Campbell is an obvious wackadoodle (with his spurs and swastikas), he does make some relevant points about American poverty in Chapter 5, Sections 55-8. Like all the potential villains in this novel, Campbell is more than just evil. He truly seems to feel principled outrage at the common assumption Americans make that if they are smart, they should be rich... and if they are not rich, they must be kinda dumb.