Not OP, but I could see that being the case...if that wasn't what looks to be concept art as the way she looks is different in the serialization. Who knows if that is true or if it would have been something in MC's mind.
Anyways, I dunno... I haven't read a chapter of this in a long while because it feels like it won't actually go anywhere or solve the mystery any time soon. But, based on this omake chapter I still question if she is actually seeing anyone. Our MC has noted he hears the noises most nights and throughout the whole night, but here we see her in her clothes (which if she was sleeping with someone she might likely just sleep naked), she has one pillow on her bed, her phone is charging right next to her head. No indications that anyone else would even be anywhere there. Of course that isn't conclusive about not having another person, but who knows.
I would LAUGH if our MC is such an unreliable narrator that he is making up these noises in his head because of some delusion he has about her lol.