He picked up on how she changed her cigarette brand, and how it matches her guy friend's cigarette brand – he…
There's something I noticed about the flowers on the posters too. The bullies' flowers are pointed downwards, while the flowers on the protagonist's side are pointed upwards. Like you said, his poster has white flowers, the same color as the characters that support DongEun, however those same flowers are pointing downwards.
Idk what to think about it, perhaps it's means that his role in the story is more complex.
Apparently, Eunkang and Chawon's families are all going to be intertwined. First there's her sister and his brother-in-law, now it looks like they're implying something related to her uncle and his sister.
Idk what to think about it, perhaps it's means that his role in the story is more complex.