The song plays whenever there's an emo scene. The singer's voice is so Korean and for once I thought it was a Korean song, but I understand Japanese so I did realize it's a Japanese song from the start. It is called Friend, by Tamaki Koji 玉置浩二.
This song is played numerous times in the drama and every time it gave me the feeling of immersion into the drama. If you understand the lyrics, you'll realize that it's a pretty mournful and emotional song. For me, I wished we could be more than friend, but it seemed like we can just be friend, like the song, "Fingers, hair, voice are becoming cold, even the fragrance of our life is passing away".
The key of the song (A major) is pretty low and that suits my comfortable vocal range. Yet I was really in the emotional state, so I was thinking maybe I could record a cover of me singing and playing the keyboard for the first time, which I did and I have to disclaim before hand that I don't have a decent voice, so sorry if your ears are tortured, but please enjoy.