Jimmy is a man who works at a drugstore.
Jimmy is first seen in the background, working, while Kevin was asking the drugstore clerk about whether or not the toothbrush was approved by the American Dental Association.
After Kevin begins backing away without paying for the toothbrush because of being scared by Marley and then turns and heads out, Jimmy was told by the clerk to stop Kevin. When he sees a police officer, Jimmy yells "Hey", and a police officer looks up to see him (initially puzzled at what was going on). Jimmy then points out Kevin and says "Shoplifter", and the police officer heads off after Kevin. Jimmy isn't seen in the film afterwards.