The books are frustratingly light on details, but seem to agree on a couple things. Firstly, spirits are constructs of magic. However much they may resemble a person, they are not people in truth. Concurrently, they cannot grow or develop in the same way people can. Their nature is an intrinsic part of their being. Finally, and most importantly, they cannot be directly seen by mundane folk. There is no trustworthy information about fighting them, although Shroud's bullets passing through cleanly seemed fitting.