Does “spirit” exist? If so, what is it?
I understand that you want to avoid straying from the specific notion of "spirit", but it's important to consider that it falls into a category with numerous other fantasies and delusions in that all of it is unknown, based on emotions such as fear and anxiety, assigned characteristics cherry-picked from natural occurrences, based on concepts and principles subscribed to by primitive humans who thought that the brain was in the chest where we now know the heart is.
There's never been any reason, outside of heightened emotion, to assert that anything invisible or intangible can be described with elaborate detail.
There's nothing wrong with assertion, and I don't see a problem with the assertion that something has never been demonstrated. If you want to argue the existence of something, it might be best to begin with some evidence of a replicable qualitative occurrence of it in reality. Otherwise we're talking about nothing as though it's something.
It's important to consider all fairy tales, not just one specifically, because they're all derived from similar heightened emotions and states of mind, such as fear of predators, fear of death itself, or fear of not having lived fully, etc.