Okay so, initially I thought after Battleground it was a closed case the Mac was an angel. The way he says "you'd hurt yourself" to Dresden, when the latter tried to use his Sight, sounded very similar to the way the angel police officer in Un-Chicago taps Harry's forehead and forcefully closes his Sight, while saying "you'd hurt yourself", but it isn't outright confirmed. There are things other than Angels that would harm a mortal to look upon, Shagnasty, the Dragons (capital D), and likely Odin (considering Gard's comments at the end of the Battlegrounds).

Researching this, I came across a theory that Mac is Merlin: (https://www.reddit.com/r/dresdenfiles/comments/4a6eki/spoilers_all_macs_true_identity/) which I love but I'm not certain if it's on the nose.

Re: The "you'd hurt yourself", comment. If Mac was Merlin, he'd be a normal, long-lived (even by wizard standards) mortal. He wouldn't harm Harry if LOOKED upon. However, some versions of the Arthur myth have Merlin being a Cambion, a half devil, thus explaining his power. If Mac (which backwards is Cam, and could be the first part of Cambion, going full tinfoil hat), then that could be it, but there is another fly in the ointment, Ethniu makes a comment that Mab wept bitterly for Merlin after the Conqueror (Hengst and Horsa? or maybe William in 1066) came, and I got the sense Mab and Merlin were lovers at one point, yet Mab and Mac (which are interesting names seen side by side like that) are cordial and respectful for each other. True, Mab is professional enough (for lack of a better word) to deal with Mac cordially, but would she wish him "honest customers" if they were ex-lovers?

WOJ is that we've "been told what Mac is, but you'd have to be pretty nerdy", and that he's not a greek God, or Scion, so that rules out Bacchus or similar entities like Pan, further Jim said that had "already told us who" Mac is, not WHAT, so Mac is a specific entity, not a generic thing like just a Duergar, or Brownie. So, he could still be a God from a non-grecian pantheon, there are plenty of beer related deities, most of which are associated with hospitality, which fits Mac to a T.

Going WAY out on a limb, Gobekli Tepe (what is perhaps THE oldest temple in the world) houses jars lined with what are thought to be the dried remnants of beer, and animal remains that seem to indicate the beasts were slaughtered and cooked (https://www.smithsonianmag.com/history/gobekli-tepe-the-worlds-first-temple-83613665/), and what are Mac's two specialities? Beer, served at warm temperature as it was for most of human history, and steak, cooked over a woodfire. Could Mac be one of the oldest Gods? Or perhaps an angel worshipped as one?

However, WOJ is the gods were put on the side line except for a select few who had to give up their immortality to continue being active. If that were true, Mac would likely have had to do what Odin did and start picking up mantles to stay alive. Now, we don't know for certain that he DOESN'T have any mantles, but if he does we haven't seen him using them, so the "forgotten god" hypothesis might not be accurate.

We know Sharkface refers to Mac as " watcher" which might imply he is one of the Grigori, the watchers who took human women as brides and fathered the Nephilim. Sharkface also refers to Mac as "empty", which might mean he lost his Grace at some point, see above. However, if Mac was Grigori, he would have Fallen, like Uriel would have if Michael misused the loaned Grace. Mac doesn't give of the atmosphere a Fallen like Ursiel or Namshiel does. The only thing I can think is that falling in love with a human woman, his punishment might have been lighter. And one of the Outsiders says Mac is meddling when he is forbidden from doing so. This also supports the Angel hypothesis, but suggest Mac isn't "Fallen", as the Fallen DEFINATELY meddle whenever they can.

So, is he an Angel? God? Something else? I'm sure this has been discussed ad nauseum but I wanted to get my 2 cents in.