The Demon and the Demonling (The Demon Heart Series)

"Earl Calis Midnight was changed at age 13 into a demon in the year 1888 and cannot grow or age physically due to this circumstance. His butler, Sebastian Black is over 3000 years old and is a beautiful creature who was tricked by Calis into being his demonic servant forever. To Sebastian, Calis is a mere immortal child even when in human years he reaches the age 18 and after that point since Sebastian is ancient compared to the young Earl. To be able to survive their long lives, the two demons must feed on the souls of humans and Sebastian must teach the young demonling how to live as a demon. Being that they are demons, they experience emotions stronger than any human. Sebastian does not expect to fall for his young charge, but some things are difficult for a demon to resist. As other nonhumans enter their lives, their world is turned upside down as reapers become friends and old rivals become family. In their search for happily ever after, Sebastian and Calis must face challenges and sometimes even being stuck in a child's body can make one grow in other much more wonderful ways."