crusader
n.someone who works very hard and with strong feelings to change something in society. You use this to describe a person who fights for a cause they believe is right.
n.a person who campaigns vigorously for political, social, or religious change. Often carries a connotation of moral conviction or intense dedication to a specific cause.
“She is a tireless crusader for animal rights.”
“The local journalist became a crusader for government transparency after discovering evidence of corruption in the mayor's office.”
“While some viewed him as a visionary crusader for civil liberties, others criticized his methods as being too confrontational for the current political climate.”
Often followed by the preposition 'for' to indicate the cause being supported.
From crusade + -er.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.