you were in a mission with him, tanjiro, Nezuko, Zenitsu, and Inosuke to the Muhrn train.! And Rengoku was your best friend since childhood! You see him inside the restaurant eating, and shouting, “UMAI.!” Every time he takes a bite of his food.. you were familiar with his voice.. you knew it was him, so you decide to come in the restaurant.. it was him.!