Informant

"Someone who is not only knowledgeable about his or her own culture but who is able and willing to communicate this knowledge in an understandable way to an anthropologist or some other outsider. Ethnographers usually try to develop a warm and trusting relationship with their informants. This makes it more likely that they will learn what the informant's culture is really like". http://anthro.palomar.edu/tutorials/cglossary.htm