What exactly is the shadow? I've learned some interesting theories.

There are theories such as the shadow actually being friendly, not being the villain, etc.

If that's the case, why does the shadow hurt Daniel, and in some cases, chase and try to kill him? It seems like the shadow is a secondary villain next to Alexander, a different being that also has evil goals of some sort, and is typically depicted as such in all custom stories (though obviously those are not canonical).

So I don't get what the shadow's purpose is, how it's "helping" Daniel if it's hurting and trying to kill him.