Barry Duffman, more commonly known only as Duffman, is the person who advertises for Duff Beer. He is known for his catchphrase "Oh yeah!", and usually utters it at needless moments. He has also worked for the owner of the Springfield Isotopes, and various other people.
Duffman obviously prefers not to afflitiate himself with something. In Co-Dependant's Day, for example, he said he was Jewish. However, in the same episode he supported Hitler's Reich. He also believes that Jesus is the ultimate role model for people, as in one episode he asks himself "What would Jesus do?".
Duffman speaks in the third person, often saying "Duffman is {insert any action}. Oh yeah!", accompanied by pelvic thrusts. He has written a world record book and often bursts into Moe's Tavern to advertise Duff.
Sometimes there are more than one Duffman.