Sunghoon’s song never made so much sense until the 9th episode. The lyrics may have been taken from the script. It is so touching and it feels so special. The fact that it was Sunghoon/Cha Jin Wook sang it made the meaning stand out.

Ever since the day with the unbelievable moment
In this world that is only filled with you
I’m just alive and breathing

I’ve never forgotten you even for a day
Though you get faraway like a dream
I try catching you then letting you go

I always hid this from you
But I’m regretting it
That fluttering feeling that day, my true feelings

Sometimes, to the point of anger, to the point of hatred
I get exhausted with this suffocating longing
Tears fall, I keep missing you

If it’s only one, it’s you
Even the second, it’s you
My one and only person, do you know it’s you?
I’m drawing you out
I’m shouting from here so you can hear
My last person, it’s you

Every line and every word in the lyrics is very well portrayed in the drama.  I can;t get over the chorus.