Ian Duncan is a British-born professor of psychology at Greendale Community College. Though Jeff once successfully represented him as a lawyer by exploiting the legal system, Duncan refuses to help him cheat. He also has a certain self-importance complex as made evident when one of his infamous theories was disproved. He provides free therapy to several students including Britta and has a rivalry with Señor Chang. He cheerfully antagonizes Chang over being fired and subsequently becoming a student at Greendale. At the end of the first season, he is suspended by Dean Pelton for his drunken behavior at a school function, prompting Chang to punch Duncan in the face with a roll of quarters and attacking him.
In season two, Professor Duncan takes over teaching the introductory anthropology course after the original instructor is fired for trying to strangle Jeff Winger. This assignment comes despite the fact that Professor Duncan admits tht he knows nothing about anthropology. As a result, Duncan often spends his time in class drinking alcohol and assigns his class multiple dioramas as make work assignments. In one class he spent forty minutes improvising a lecture on anesthesiology in order to impress a pretty student who had wandered into the wrong class room. The final exam for the class consisted mostly of a drunken party at which students openly praised Duncan for running "the ultimate blow off class" and Duncan announced that everyone was getting an A. This ended when Dean Pelton and a reporter arrived to observe the class and Duncan used the pretext of having left the exame paper in his office as an excuse to flee.
In the second season it is revealed that Duncan has obtained a restraining order which requires Ben Chang to stay 25 feet away from him at all times. Duncan uses this restraining order as a "force field" to keep Chang from taking his class and even to chase him out of the cafeteria at lunchtime. Afterwards, Chang gets his own restraining order against Duncan and the two seem to reconcile. When Britta and Annie have a catfight on the quad, Duncan declares "this is why I came to America" and even has his picture taken with the two. When Abed has a nervous breakdown at Christmas and hallucinates that everyone is claymation, Duncan attempts to diagnose and cure him, with the hope of getting a major book deal out of the interesting case. Over the course of the season he does seem to become closer to Jeff through such actions as counseling him over his fear of death, and watching a Manchester United match at Jeff's apartment.