Synonyms for Companion (same or very similar meaning)
WordNet sense 1 (a traveler who accompanies you):
fellow traveler, fellow travellerWordNet sense 2 (a person who is frequently in the company of another):
associate, comrade, familiar, fellow, associateWordNet sense 3 (be a companion to somebody):
accompany, company, keep companyFrom the ODE community, based on WordNetadd/edit