@MJn well said! If you think about it, Geom sees Yeonseo as a sisterly type. Specifically an older sister type. I think it's a stereotype that older sisters are usually very worried for their little brothers when they go out and sometimes the little brother has to prove that they can take care of themselves so their sisters won't worry too much for them.
In this case Geom is the little brother who came to see his big sister Yeonseo. He didn't change his hairstyle nore the way he dresses because he wants to be seen as cool and collective and someone who is able to take care of himself so Yeonseo doesnt have to worry about him.
Remember Yeonseo is the kind of person that would make fun of Geom in a comical way. So Geom doesn't want Yeonseo to se him being weak. He wants to show his cool side If that makes sense
Going on to Wooyeon, it's a little similar but very entirely different. Wooyeon after all is Geom's First love, his long time crush, an someone he so desperately desires to be with. You can tell that he puts his hair down and also wears a different type of clothing because he want to look appealing in front of his crush or rather not scare away his first love.
In other words Geom still thinks about what Wooyeon might think of him. And he doesn't want wooyeon to see the other side of him that deals with thugs on a daily. He wants Wooyeon to see the Geom from the past. The one who would cling to him, the one who'd laugh beside him. Of course Wooyeon can tell he's a bit different so there was no way hiding himself would do any good
This also signifies that Geom so desperately wants Wooyeon to see his Flaws and accept him.
OML I've said too much😅but you get what I'm saying. The author is a genius, she never leaves a plot unfinished.
