Any when I say justice, I mean the higher principle of justice, not just the physical law, but someone who believes in the higher meaning of justice, almost a nobility....but will fight to uphold it.
A person who campaigns vigorously for political, social, or religious change; a campaigner
This allows you to apply the term to passionate individual in any field or with any goal. It can also lend a hint of righteousness due to the term originating from the Holy Crusades starting in the 11th century.
I am always reminded of Batman, who is also known as "The Caped Crusader".
They are an untouchable.
The Untouchables were a group of nine U.S. federal law-enforcement agents led by Eliot Ness, who, from 1929 to 1931, worked to end Al Capone's illegal activities by aggressively enforcing Prohibition laws against Capone and his organization. In their conduct, they became legendary for being fearless and incorruptible, earning the nickname "The Untouchables".
A superhero - A person with extraordinary heroic (brave, courageous, noble) attributes (OED).
1899 Daily Mail 29 Sept. 4/4 M. Clémenceau suddenly burst out with, ‘All the world knows that Colonel Picquart is a hero, but..if Colonel Picquart is a hero, Mathieu Dreyfus is a super-hero.’(OED)
1917 ‘Contact’ Airman's Outings 211 The super-heroes of the war. (OED).
Humanitarian is the accepted term, or egalitarian..