The Circle of Secrecy is the sdecret organization that works for Grand Arch Demon Oolong. There are only a few known members, but more may exist elsewhere.
Succubus 18 was the Grand Arch Demon Oolong's right-hand woman. She current is permanently fused with Oolong, turning him into Omnibus Oolong 18.
Devilman is the one who introduced Oolong to the secrets of Demons and HFIL.