I think its a metaphor. The way she poses in the picture is sort of like what you'd do with your boyfriend. People in general can be monsters. I see it like this: You never notice evil people even when they're the ones nearest to you. You dont see them, but they see you.
I have no idea what the artist is trying to convey if anything, but I feel as though these images really are just interesting 'imaginary monsters', and not meant to represent anything more than fun imagination. Having said that though, I do think your take on it is interesting
That's @$#@$ scary. What is that, a heartless modeling itself after a Satyr? With a dash of Venom in it? Dang. This monster series 'just cuz you can't see them" are talking about ghosts and demons, too, or just specific things?