WordNet sense 1 (something inverted in sequence or character or effect):

inverse

WordNet sense 2 (a relation of direct opposition):

contrary, reverse, contrary

WordNet sense 3 (a contestant that you are matched against):

opponent, opposition

WordNet sense 4 (a word that expresses a meaning opposed to the meaning of another word, in which case the two words are antonyms of each other):

antonym, opposite word

WordNet sense 5 (not the same one or ones already mentioned or implied):

other

WordNet sense 6 (characterized by opposite extremes; completely opposed):

diametric, diametrical, polar

WordNet sense 7 (unlike in nature or quality or form or degree):

different

WordNet sense 8 (of leaves etc; growing in pairs on either side of a stem):

paired

WordNet sense 9 (directly facing each other):

face-to-face