A government appointee who investigates complaints by private persons against the government WordNet 3.0 & Open English WordNet

An ombudsman, ombud, ombuds, bud, ombudswoman, ombudsperson or public advocate is an official who is usually appointed by the government or by parliament to investigate complaints and attempt to resolve them, usually through recommendations or mediation. Wikipedia

Government service charged with investigating complaints of maladministration or violation of rights Wikidata

An appointed official whose duty is to investigate complaints, generally on behalf of individuals such as consumers or taxpayers, against institutions such as companies and government departments. Wiktionary