Zugaikotsu no Holy Grail

is a Japanese light novel series written by Tomonori Sugihara, with illustrations by Yuuchi Kosumi. The series began publication on July 10, 2002 by ASCII Media Works under their Dengeki Bunko publishing imprint.

Synopsis
Mizue Yagawa was worried about her "work" companion-Hiroto Suga. I don't deal with myself, who is a beautiful girl that everyone looks back on, and I only care about my sister--even though I was attracted to her from the time I resumed, she just ignored Mizue.

I knew the reason. Hiroto Suga's sister was a victim of a certain incident, and the incident was famous. Since losing his fiancée in the occult-related incident, Hiroto Suga's sister has behaved as if there were an "invisible child," and Hiroto was eager to take care of it.

In front of Mizue, such Hiroto was disappointed when he heard the name of Baphomet, the object of worship of the occult followers who attacked his sister ...! Dark fantasy is here!