So lets confirm things,,,
This girl's name is Seohyun and she was the girl who used to hungout with Moonyoung (ML).
So what do we know about her?
She saw the letter and the contents of it, but the person who took the letter is Jonghyuk (annoying glasses dude).
What are maybe assumptions that I think is true?
I think she still likes the ML.
In the past chapters, Moonyoung asked her if she was the one who deleted the voicemails and phonecalls from the MC. She said it was Jonghyuk. But I think otherwise. I think she was lying on that part, but who knows what.
So who is Geonwook?
I don't know. But I know a guy named Gunwook is the ML's buddy and the boyfriend of the MC's ex-best-friend.
(Seohyun and him, are not dating I think. I never even saw them together in the past chapters.)