complement has 2 distinct synonyms spread across 2 of its 7 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a word or phrase used to complete a grammatical construction
a complete number or quantity
“a full complement”
number needed to make up a whole force
“a full complement of workers”
something added to complete or embellish or make perfect
“a fine wine is a perfect complement to the dinner”
one of a series of enzymes in the blood serum that are part of the immune response
either of two parts that mutually complete each other
make complete or perfect; supply what is wanting or form the complement to
“I need some pepper to complement the sweet touch in the soup”
Synonyms, antonyms and the broader and narrower relations for complement come from Princeton University’s WordNet. Full sources and licences.