The Crane Wives - The Moon Will Sing
Fukumaru is singing the song! especially the line - I shine only with the light you gave me! because he would have been alone in the pet store if it wasn't for his loving owner （/TДT)/
https://www.mangago.me/read-manga/ojisama_to_neko/
Here's mine no matter how much I read it I never get far enough because it brings me to absolute tears. Σ(っ°Д °;)っ
I just read the first chapter but i had to write this. This needs a anime asap! i can't believe i didn't give this a chance for so long.