Really Really Miss You brought me here. This guy's voice is too hot. Hehehe. Though I'm looking actually for a male and female duet cover of this. All the duets I have found are of 2 females.
I’m re-reading the story again after 3 years. This song ‘Legend of the Koi’ appears in chapter 28. I’m now a fan of this singer, his other song that also appeared in the book, ‘Shang Ye’ is so nice too..
According to the site I came from (translation of the novel Really Really Miss you), the song is “Jinli Chao” Legend of the Koi and the singer is 小曲儿 (sorry, don't know the name translation)