I feel for Sono, now looking back. Maybe the ghost/entity thing is as just as it says it was - it's a metaphorical "life that was denied" in all possible meanings, it's just that Sono is the one that brings them together. It's terribly sad how she was neglected by her own family, just because she wasn't pretty or smart as her siblings (hence why the "child" adopted similar one-piece dresses like the ones that her sister used or the ones she wanted to use as she got older), and once she noticed Shun (whom, by the way, we only know that his high school years were "the most fun he ever had", according to this ghost/entity) she somewhat saw herself on him? and tried so hard to get noticed by him as well.
If she was different (another "life that was denied" situation), she wouldn't have acted like a hopeless puppy ("stalker" if we go for harsher words - the only time we know she was more of a stalker was when Shun got sick) towards him (or Susumu). And thaaaaat also brings me how she got attached to dudes that were acting like trash to her (the make-up dude and "Guitar"). Maybe it's true that the "girl" was also a manifestation of an "unborn child", (the initial "life that was denied" situation), as well, because - the girl looked like her, but w/o her natural hair color-, and if she would have been in another different situation, perhaps the life that she was, indeed, denied... maybe she would have given it to her - something about ending cycles of abuse.
That being said though, depending much on Susumu wasn't healthy... because Susumu also has its own issues to solve, and all that jazz.