It is important not fall into the trap of looking for a word which is the opposite of "grumbling about myriad perceived injustices, braces himself against an oppressive world."

Rather, we see "heretofore has featured a(n) ______ protagonist," and therefore look for an adjective that describes the protagonist "who, grumbling about myriad perceived injustices, braces himself against an oppressive world."

Clearly, the words are gloomy and saturnine