A note from SaHa:
"Just a note that the symbols that occasionally appear when Rin chants are Chinese in origin and don't have a reading in Japanese.
Also, "Fukoki" literally just means "Fuko Ogre". So, it's not exactly the ogre's name, but rather the way that Rin refers to the ogre."
The volume actually has more than just those 4 chapters but the other stories are random BDSM one-shots without any ogres in them.