Park Shinye's character is being paranoid(delusional) everything she's seeing is all in her head and she keeps questioning his faithfulness and Lee Honggi's character is 'insensible' to her feelings and didn't sense something was wrong. He's singing about how he wished he could've paid more attention to her and how he regrets not being able to help her at the time. I think that she committes suicide, and that's why he regrets it.
Peacock feather images are flashed, i read somewhere that peacock feathers represent death in Korea though i am not really sure.
Lee Honggi - Insensible