Cowl Theory
Okay, I tagged this spoilers all so that comments can be a free-for-all. I know Cowl theories are a dime a dozen, and none of us really know for sure. I’m in the Simon Cowl camp personally, but this observation doesn’t necessarily require that view.
When Harry and Cowl meet for the first time (Bianca’s party doesn’t count), Cowl acts as if he is someone who knows OF Harry, but doesn’t necessarily KNOW Harry. It has more of a “known by reputation” vibe. However, he also takes great pains to hide his face and maybe mask his voice. If we assume that his reaction is genuine, then that necessitates the question, why work so hard to hide himself from Harry? Harry won’t recognize him.
My thought is that he’s worried Harry will describe him to someone who can identify him. Someone like Ebenezer or a senior warden/councilmember. Obviously I think that supports the Simon theory, but there are probably several other candidates who fit the bill. It does seem to rule out some of the Justin/Dark Harry/Eb theories though.