Remember, even if you’re with someone else, that I was once by your side.

Songs used: Oasis-Beast

                     I'm in love- Narsha

                     Tears-LeeSSang

                     When I miss you- Beast

                     Because I miss you- Jung Yong Hwa

                     That i was once by your side.- Kyuhyun

So as the title suggests, That i was once by your side is the most important song.