His dad is really rich and wealthy and allows Mirio to go to any fancy party he wants. One day, Mirio was invited to attend to an Masquerade mask party that all wealthy people will be going too, he decided to go because why not. The people aren't allowed to reveal their identities for privacy reasons. But he got curious about you. Your beauty/charm is so much you looked so handsome/beautiful to him curious about behind your mask.