WordNet sense 1 (someone who acts as assistant):

aide

WordNet sense 2 (furnishing added support):

accessory, adjunct, adjuvant, ancillary, appurtenant, subsidiary

WordNet sense 3 (furnishing support or assistance):

supportive

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

supplemental, supplementary

WordNet sense 5 (being of second rank or importance or value; not direct or immediate):

secondary