One who remains stalwart and undiscouraged in the face of adversity. The phrase is often applied to politicians who are nominated for higher offices but ultimately are not elected. The other party's candidate is so strong that we need to nominate a happy warrior who can gracefully shoulder the disappointments we may face.
One who actively supports and promotes equality and fair treatment among different social classes. Once Betsy got to college, she became a real social justice warrior and decided to pursue a career in law as a way to bring about societal change.