He knows, that's why he became a psychiatrist in the end.
Wait he became a psychiatrist?...I thought he was there to visit him time to time.
Yup! It's in the Creator's Note, Namsoo majored in Psychology. He is also wearing a white doctor's gown in the timeskip where the twins are living together.
OMGGG!! THAT MAKES ME FEEL A WHOLE LOT BETTER I CAN DIE HAPPY NOW I didn't know all that
Does Hyeongoh find out that he is his brother or is it still only Nam soo who knows they are brothers at the end?? I can't tell and it's making me so sad! Hyeongoh finds out too right??