"Wisp" allowed interspecies breeding between humans and the supernatural.
The world is a precarious mix of purebreed demons and 2nd/3rd generation ghouls.
These two species coexist in the landscape of humanity...

Kensuke lived peacefully in the small town of Wakahime.
Things were starting to get serious with his girlfriend Miyuki.
One day he invited her to his home...
In the middle of asking her to marry him, he was murdered by a demon frothing with "wisp".

When his spirit was reincarnated, he was no longer human.

[From DLsite English]