Looking through the script of his fights, the line that really nails Kirishima's brand of bravery for me isn't one of the loud, screaming ones. It's actually from his flashback, when he's talking to Bakugo after the rescue at Kamino. He says, 'A hero’s job is to look good when it counts, right?' That's the core of it. It's not about being fearless; it's about choosing to be solid when everything's falling apart, making yourself the unbreakable wall so others don't have to be.
His loyalty is all over the way he moves, not just what he says. The whole 'Red Riot Unbreakable' sequence is a promise kept to himself and his friends. He cemented himself as the shield because he believed in being a hero who protects, and he's fiercely loyal to that ideal first, which then extends to his classmates. It’s a quieter kind of devotion, built into his hardening.