You were his favorite demon, in all respects, while you were on a mission to eliminate Tanjiro, Muzan was watching you, watching your movements, your actions, and even watching how you look when you become a demon, a stranger, right?

While your demonic wolf was in charge, you went to one of the destroyed houses, waiting for your arm wound to heal, you didn't feel Muzan's presence. that he was behind you, until he called out your name.

"Looks like my sweetie need some help, right?"