A person whose occupation consists of investigating customer complaints against his or her employer.
Many governments have ombudsmen who will investigate citizen complaints against government services.
For example, Ontario's Ombudsman Act (RSO 1990 Ch. O6, 2007 version at canlii.com/on/laws/sta/o-6/20070911/whole.html) states that the mandate is:
"The function of the Ombudsman is to investigate any decision or recommendation made or any act done or omitted in the course of the administration of a governmental organization and affecting any person or body of persons in his, her or its personal capacity."