Board Thread:Legitimate Canon Questions/@comment-25176701-20160611021923/@comment-5182725-20160620172216

About a Banshee being a supernatural creature or not. I also thought they cleared it up in an interview as well (either with Holland or Jeff) that Banshees only have access to the supernatural. It's the very reason why Lydia has never had a problem with passing through Mountain Ash. While she's not human, I'm guessing that she doesn't really count as a "supernatural" because she can't shift and she really doesn't have a form that could be dangerous to others (even if she has abilities.) At least where being able to cross Mountain Ash is concerned. If anything she's like in the middle.

It does make sense when you think about it, the only creatures we've seen have a problem with Mountain Ash (that weren't Chimeras or a Hellhound) are those that have some sort of secondary form (counting Kira's Aura, which so far a Banshee doesn't seem to have.)