fellow traveller
From Longman Dictionary of Contemporary Englishfellow travellerˌfellow ˈtraveller British English, fellow traveler American English noun [countable] PPPsomeone who supports and agrees with the beliefs of an organization, such as the Communist Party, but does not belong to it – used to show disapprovalExamples from the Corpusfellow traveller• My fellow traveller was going to a reunion with school friends at Torridon.• It is as a stranger that I greet my own self, and see it as an unknown fellow traveller through time.