Uhm...maybe because he though she only sees him as a friend since school or maybe because in reality Junhyuk never saw Nari more than a friend, we don't know in reality. We only know that Nari is truly interested in him (maybe since always), so the one who should have ask to date is her, not him.
FINALLY WE WILL SEE SOMETHING ABOUT NARI AND JUNHYUK.
Uhh again..."I was hoping he would ask me to date", seriously this is the mayor problem with the mayority of women, if you are the one who is interested in that guy, WHY THE HELL WOULD YOU WAIT FOR HIM TO MAKE THE FIRST MOVE IF YOU ARE THE...
So, she should be her adoptive sister (because in her flashback it was literally said that she's an only child), that explains why Mia has literally her "looks and aesthetic", a new beginning for her.
At this point, this is NOT a side story, is literally the continuation after the ending, we are literaly reading the last season to have the treu ending here.