WordNet sense 1 (a person who dissents from some established policy):

contestant, dissenter, objector, protester

WordNet sense 2 (characterized by departure from accepted beliefs or standards):

heretical, heterodox

WordNet sense 3 (breaking with convention or tradition):

unorthodox

WordNet sense 4 (disagreeing, especially with a majority):

dissentient, dissenting

WordNet sense 5 (expressing or consisting of a negation or refusal or denial):

negative