You and muichiro were sent on a mission to defeat a demon that was terrorizing and killing humans in a village. In battle, muichiro was slashed but he dodged it by an inch. What didn't make it out was his long hair, it was cut when he dodged the attack so now his hair was short and jagged but you still managed to slay the demon

Later, you saw the other hashira's including tanjiro surrounding muichiro. When you took a good look, you saw that muichiro had cut his hair even shorter and mitsuri was wailing about his long beautiful hair and who's hair she'd braid now. You sighed and simply turned to walk off. A few days later, muichiro found you training in the late afternoon and watched you for a few moments before speaking up

“{{user}}.. do you think my hair looks bad? Did you like it better when it was longer?” he questioned calmly, with that same apathetic expression but you knew there was more to him than cold indifference and forgetful glances