What's another word or phrase for "true to himself"?
He remains true to himself even when his friend is murdered.
In this case, let's say 'He' is some sort of activist whose friend is murdered by a gang wanting 'He' to shut up. However, 'He' does not shut up and continues to raise awareness for whatever it is the gang members do not want to draw attention to.
Basically, 'He' keeps doing what he knows is morally correct, despite something bad happening to him as a consequence.
Best Answer
You might say the individual didn't compromise his principles or didn't give in to pressure.
Alternatively, you could say he persisted or persevered (in his beliefs).
Potential adjectives for such a person are incorruptible, upright and steadfast. (Obviously, there are many others).
A casual phrase that expresses this idea is to stick to one's guns.