Word for not letting wrong pass unchallenged.

I am developing a novel, and I'm trying to pin down some traits of my character. I have one that I can't seem to define though, and I was hoping someone here would be able to help me.

I have a character who does not let wrong pass unchallenged. If he sees wrong, falsehood, or evil being gotten away with, he will not sit idly by and say nothing. If he sees a problem that is going unnoticed, he will try to fix it, even if doing so is difficult and painful.

Is there a word for this trait?

Goodie-two-shoes, White Knight, Heroic!

In DnD this would be Lawful Good/Lawful Evil depending on application.

Heroic, particularly 2b.:

Full Definition of heroic
1: of, relating to, resembling, or suggesting heroes especially of antiquity
2: a: exhibiting or marked by courage and daring
   b: supremely noble or self-sacrificing

My thought is that a word comparing him to a mythical or legendary figure known for those traits might be best...similar to "solomonic". The figure that comes to mind is Horatius at the bridge, determined that none should pass.