Kai is Kai. He does not have surnames nor did he want to have one. 

Living a life on earth is what he never expects to happen because he knows; on the other side of the world, there is someone waiting for him. 

On 1894, he lives in a place where everything is white and green. The place was named after Surbaynian for a reason. Every single thing that will be happening to the kingdom is written in the book where it was hidden somewhere in Surbaynian but no one seems to find. It was gone years ago even before his birth. 

But someone...

named Sehun with surname Oh, knows of the book whereabouts and it causes him to bear a demon's child for a thousand years.