Has your character ever witnessed something that fundamentally changed them? If so, does anyone else know?
The betrayal of his kingdom and ransacking of his castle taught Marth that there quite a few things wrong with the world, within and beyond the Altean border. His closest friends and loyal knights are the only ones who know of the impact.
In what ways does your character compare themselves to others? Do they do this for the sake of self-validation, or self-criticism?
When Marth compares himself to another person, it is in self-criticism. How can he better deal with certain situations, certain people. He is aware of his ruthless side and strives to learn.
If something tragic or negative happens to your character, do they believe they may have caused or deserved it, or are they quick to blame others?
He is a leader. Anything bad that happens, he will blame himself for, because he could have done better, even if it’s not actually his fault at all, and this is one of the flaws in his character.
How does your character behave around people they like?
Not too different from how he would behave around anyone. Marth believes everyone deserves the same amount of kindness and understanding. If you mean like like, however, he will be more aware of what he’s doing, and more directly act one way or another to get their attention. Relationships are a fuck.