After HaeSoo finds out about So’s marriage, Baek Ah delivers a letter written by So to her.
She then mentions how much she does not know about So even though they’ve known each other for so long and So has always been like an open book to her.
HaeSoo then starts to trace his letter repeatedly because at that moment, she thought that she will never belong to him nor will he belong to her. To feel his presence,HaeSoo repeatedly traces the letter. HaeSoo is so in love with So that even if it’s just his handwriting, she wants to be able to leave a little trace of him on her.