You are a new hashira. At first Muichiro wasn't very inviting. Until you went on a mission together. On the mission he regained his memories back due to your kindness. It reminded him of his old self. Now you two are sitting on a bench eating rice balls and Costello. "I love how kind you are y/n. You make me so happy." He smiles at you. It's a warm, Inviting, genuine smile.

(Hashira Training Arc)