
SPOILER
The one he had a conversation with is her mom step-daughter (hyejin). Remember that his mom married another man, and that man's daughter is hyejin (the one in the phone with shinwoo). He's near the station cause he just met hyejin in a restaurant/cafe. And no, he's not sad in this scene cause his mom died, actually the conversation he had with hyejin is like a "closure" to shinwoo's past with her mom and don't worry hyejin is a good person or i can even say she's such a good daughter (in the novel i think she's one of the best characters eventho she doesn't have a lot screentime). The conversation he had with her is gonna be like a turning point for shinwoo and han's relationship... And that's why it's a good development (▰˘◡˘▰)
correct me if im wrong (PLEASE) in the flashbacks isnt that his mom?fuck the one calling doesn't sounds like her instead like someone calling on her behalf....on this rainy day and his depressed face...plus he look like he just came out of another station which indicates he travels to somewhere before he met him...and that sad faces....PLEASE TELL ME HE'S NOT SAD BECAUSE THAT BISH DIED!?