Auxiliary (adjective, noun)
Providing supplement or additional help and support, extra, spare, reserve, back up, fall back, supplemental - not necessarily in a lower status.
Ancillary (adjective, noun)
Synonym: collateral; secondary, subordinate (literally "lower number" sub ordinate)
Corollary (noun, adjective)*
Proposition which follows from one already proven.
Collateral (adjective, noun)
Synonym: ancillary; parallel, side-by-side, accompanying, concomitant (literally "on the same plane" co lateral)