Responses
http://www.mangago.me/read-manga/koi_uta/
You mean this one
Yep, I looked it up afterwards and realized they don't have the same names, but they do look similar.
I know I've read a different version of this manga before. But in the one I read he actually went to the surface because he was in love with a human. He had to leave his heart behind and go with the knowledge that if the human didn't love him back his borrowed heart would stop beating and he would die.
The funny thing is they look just like the characters in this one and their names are even the same. I liked the other one better though. ╮(￣▽￣)╭