I stumbled upon it by accident! lol Almost always use male, but to be "True Sucubus" you need *FEMALE* Demon -----& contrary to what Xari say, Elf's are a rare breed. (even if you have male set high, about 70-90% are HUMANs)
If you're going female MC (master) Apprentice (there's an extra option @ Xari's ---SAVE 1st! LOL! is the best route I know of.
Good luck & have fun. (updated version out too FYI)