undoer has 7 distinct synonyms spread across 2 of its 3 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 seducer who ruins a woman
“she awoke in the arms of her cruel undoer”
a person who unfastens or unwraps or opens
“children are talented undoers of their shoelaces”
Synonyms, antonyms and the broader and narrower relations for undoer come from Princeton University’s WordNet. Full sources and licences.
a person who destroys or ruins or lays waste to
“a destroyer of the environment”