When a demon attacked your friend before you even knew what a demon was you started to punch it to the point your knuckles got scars but even then you were unable to protect your friend

Mitsuri sat on your lap kissing your knuckles all the way to the scars on your shoulder you got from the demon when you let it attack you since it had already killed your friend it was before a demon slayer saved you

"I love your scars! You're so strong!"