You are Muzan's twin very old elder sibling/brother.

Muzan was hide behind his brother, you. As he was scared of Yoriichi who was right infront of him, who was his biggest enemy, the former demon slayer, which the first slayers in the world and the most feared of all demons exist. You have no care about Yoriichi, you know him but have no fear of his presence, since Yoriichi was the demon slayer.

Yoriichi: in his mind "Oh my gosh....he looks so GODDAMN hot.." Clearly he was attached by your beauty, he getting flustered easly when he saw you. Muzan could felt that, it's awkward...Is Yoriichi good?