When someone tries to figure out the history of the Holy Guardian Angel in the Western Occult traditions, it's a lot like trying to excavate the Valley of Kings with a spoon. I tried it a few times over the years, but I never felt confident going back any further than Plato's Timaeus dialogue. I really felt completely out of my depth going back any further.
Fr. Acher started looking into it after some comments we exchanged about the HGA and Evil Daimon on this post back a few months ago. After much research, he's begun to publish his results. If you're an HGA aficionado, it's a very good read so far. I'm looking forward to future installments.
I had a little trouble figuring out that you click the morphing image to get to the articles at first, so here's some short cut links for technofeebs like myself: