WordNet sense 1 (an assistant subject to the authority or control of another):

subordinate, underling, foot soldier

WordNet sense 2 (a company that is completely controlled by another company):

subsidiary company

WordNet sense 3 (furnishing added support):

accessory, adjunct, adjuvant, ancillary, appurtenant, auxiliary

WordNet sense 4 (functioning in a subsidiary or supporting capacity):

supplemental, supplementary