Responses
Because of how homogenous the manga/manhwa art style is, the only difference you can tell at first glance is a character's hair. If you have two leads with the same hair color, they might be confused as siblings (I read a straight manga, I forgot the name, about this married couple and they had the same colored hair and I kept thinking that they were siblings-)
Like almost all stories, the couple has one dark haired person and the other one light