The story has three main elements: 1) An oppressive urban setting that the protagonist finds constricting and disorienting; 2) the protagonist, the half wit, a beautiful, naive man with an angelic like innocence, a blonde, green-eyed Adonis; and 3) his love interest and pseudo-antagonist, a mysterious older woman; a dark Persephone.
I would like the design to be either expressionist or surreal to capture the mood of the story--oppressive city contrasted with man who is beauty and light, and the voluptuous woman. She has long jet black hair that reaches out to ensnare him, helps to hide her nude body. Her face should either be turned away from the viewer or in profile that is mostly obscured by her hair.
I am literally thinking of Michelangelo's David when I describe the character in the book, bu a modern version. Curly blonde hair and green eyes, passive facial expression like the statue. Fully clothed on the cover, looking away from the female character. You could even manipulate a photo of the statue--put clothes on, get the right pose and style to the illustration so that it looks like a real person. I added more images for inspiration on this.
These colors and visual elements are important parts of the story.