sympathetic has 10 distinct synonyms spread across 3 of its 6 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. Its one recorded opposite is unsympathetic.
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.
expressing or feeling or resulting from sympathy or compassion or friendly fellow feelings; disposed toward
“sympathetic to the students' cause”
showing or motivated by sympathy and understanding and generosity
“was charitable in his opinions of others”
(of characters in literature or drama) evoking empathic or sympathetic feelings
“the sympathetic characters in the play”
relating to vibrations that occur as a result of vibrations in a nearby body
“sympathetic vibration”
2 of the 6 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for sympathetic.
Synonyms, antonyms and the broader and narrower relations for sympathetic come from Princeton University’s WordNet. Full sources and licences.