At some point, a fair assumption can be made that we –collectively– have all idealized a person to the extent that the way we have crafted them in our heads does not correlate with the person they are in 
real life. We create an image of this person in our minds, oftentimes with unreachable standards. The disappointment that can be felt when we acknowledge that these people are simply just people, and not the romanticized version, is justifiably heartbreaking. So, when our narrator decides to not see Isabel for who she is and instead leaves with the fantasized version still intact, it almost seems warranted. What if her physical appearance does not live up to his standards? What if this completely changes how he perceives her as an individual? He has an unmatched connection to a girl he has never seen before and knowing that seeing her would significantly humanize her, he decides to opt-out.
Perhaps he is aware of the fact that Isabel is, in some ways, nothing more than a fantasy. He thinks about her constantly, always eagerly awaiting the next time he can talk to her. While Wolf begins as the centerpiece of our narrator’s obsession, it’s obvious that Isabel soon becomes his secret pipe drea
m. His fixation on her causes her to become something not obtainable, and the moment that she is, he doesn’t want that non-obtainability to cease. The chase ends. The magic is lost.
And when she ultimately leaves and he loses his one chance at uncovering what she really looks like, his fantasy of her can continue She is still unknown to him, still unreachable. She is still a dream that he can disappear to.
I thought of Isabel as a ghost, but you have a wonderful point here – she could be a dream. It fits the bill entirely, yet could I add that it could be a lucid dream or disassociation from the world?
I agree that it is possible that she was a dream. The whole story revolves around reading and fantasizing about other places and people. His need to be around Isabel, even when she is not there, is much like becoming obsessed with a book.
At first, I didn’t consider Isabel to be a figure in a dream, but I am beginning to consider the possibly. I wonder if that is why she is rarely around others, as she doesn’t exist. It is possible that she was just a hallucination for him, similar to how people with certain mental health conditions hallucinate people, sounds, and situations.
If anything, I consider her to be more of a hallucination than a dream. If she exists also to Wolf and her mother, maybe they also have the same condition that causes them to hallucinate her existence?