charles i has 7 distinct synonyms spread across 3 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.
as Charles II he was Holy Roman Emperor and as Charles I he was king of France (823-877)
son of James I who was King of England and Scotland and Ireland; was deposed and executed by Oliver Cromwell (1600-1649)
king of the Franks and Holy Roman Emperor; conqueror of the Lombards and Saxons (742-814)
Synonyms, antonyms and the broader and narrower relations for charles i come from Princeton University’s WordNet. Full sources and licences.