What do you call someone who doesn't back down? [closed]

I'm looking for a word that describes a character in an essay I'm writing. I need a word for someone who doesn't back down, something like "brave" but not quite, more like "courageous". This person is protective, and stands up for others no matter what. In one scene, a group of people are attacking a student, and this character stands up to them and gets them to back off. Even when everyone is against them they stands up for what is right, no matter the odds.

Edit:

The character is a teenager, if that helps at all.

Edit

A teenager with principles, a person who defends the underdog, a person who's not afraid or ashamed to stand up for what's right.

Solution 1:

It sounds to me like they're stalwart - combining both the meanings of "valiant" and "firm, steadfast and uncompromising" (reference). Often used in the phrases stalwart friend and stalwart supporter.

Solution 2:

tenacious

adjective
tending to keep a firm hold of something; clinging or adhering closely.
"a tenacious grip"
synonyms: firm, tight, fast, clinging; More
not readily relinquishing a position, principle, or course of action; determined.

Solution 3:

Sounds like your character is resolute.

The word connotes both bravery and conviction.

Solution 4:

Someone who, even when everyone is against them, continues to stand up for what is right, no matter the odds, sounds indomitable to me.

indomitable adjective: impossible to subdue or defeat. "a woman of indomitable spirit"; synonyms: invincible, unconquerable, unbeatable, unassailable, invulnerable, unshakable, unsinkable. see, Google