Responses

I don't think it's his twin brother because when Seo In went to the birthday funeral of his brother .... His granny told someone of the family that you can't blame Seo In to try having a good relationship with someone who is not related to him by blood...
So we can 100% assume that Seo In and the other brother were not related by blood at all.
So the person Seo kissed was his goddamn twin brother..?