Many years ago I was commissioned to do a series of illustrations for a science fiction book by Jacqueline Lichtenberg. At that time she was known for her "Sime-Gen Universe" series involving a dual form of humanity, in which one form depended on the other form for sustenance by a kind of energy parasitism. In this concept, unrelated to Sime-Gen, a human pairs with an alien to find living crystals that endow the wearer with techno-magical powers. I featured another one of these illustrations in a 2010 posting here, and you can click on it for details if you want to.
The character here is the alien partner of the human. Throughout most of the book the alien seems to be androgynous or sexless but by the later chapters the alien turns out to be a female who is in love with the male human. She is humanoid but I'm not sure they were physically compatible.
I did a whole series of these black and white illustrations, which Jacqueline Lichtenberg eventually bought. The book was never published and the whole project disappeared into obscurity, remembered only by the author and myself.
Original is ink on Bristol board, 8 1/2" x 11", September 1994.