G-Kyuu is a monster like creature that Agarta encounters & fights while searching for the serpentine dragons.
Because Agarta must have her own consciousness removed in order to reach into another persons mind, the G-9 is responsible for returning her "ego" (consciousness/identity) to her after she has reached the deepest part of her clients consciousness so that she can remember who she is & destroy the dragon. In order to regain her ego Agarta must fight & kill G-九.
It is never clearly explained what G-9 are exactly however, their appearance is clearly non-human & there are a number of them. Agarta comments that 3-5-10 told her that they are her children, but she has never tried to understand what that means though she does seem to feel some compassion for them.
