tip for all: Korean default is formal, at first meeting unless the person is rude.
formalities follow age for the most part.
If in professional setting, the formalities might never be dropped (e.g. between Bong Seon Hwa and Jeong Jin). Yes they had kissed (broke irmar’s rule right there).
It is also a balance betweeen that and what makes sense in target language. Someone said in German you never talk to your father in law formally.
In Korean you never ever ever ever use “you” unless all formalities are dropped. For the sake of the English we put you. Also superiors who are family and teachers refer to themselves in 3rd person (Dad thinks… Mom thinks… Teacher thinks…Grandpa thinks… Aunt thinks…Oppa thinks (yes in the lover sense) and the older brother sense too)