Big-D is Ea-Nasir
Big-D seems like the type of person to sell low-quality copper.
Also, for some actual theory, Ea-Nasir is widely regarded as having achived figurative immortality, similar to Gilgamesh, though the fact that his buisness complaints survived to the modern day. By World of Darkness lagic, this immortality could be literal in the form of him being a Mummy, something that has been hinted at with how much they've been mentioned in the background (Big_D bringing up Mummy Death Masks, Marckus wonding if people not in the know about the supernatural have figured out if Mummies are real yet, the reason for the secret entrance to the Arcanum Archives being sealed being loosely because of an Egyptian Mummy), and would also make be tonally consistant with the Emperor being a giant mummified corpse.
Speaking of Gilgamesh, the reason why Big-D reacted so strongly to Gilgamesh killing Akira Cross' mother in Boy's Story could be because Gilgamesh is Babylonian. Ea-Nasir is from the City of Ur (also: possible connection to Big-D mentioning the Game of Ur), which was sacked by the Babylonian Empire around what would be within Ea-Nasir's lifespan. If Big-D is Ea-Nasir, this, compounded with likely centuries of hunting the supernatural, likely triggered some form of PTSD.
Also, to counter QosmicVoid's point on why Big-D can't be from Ancient Mesopotamia, he could have just called the Game of Ur by it's modern name instead of by what it was called in Ancient Mesopotamia because he was talking with Kitten, and, because he was genuinely suggesting the game, refered to it in a way he knew Kitten would probably recognize.
(This theory was cross-posted from Reddit. Both posts were by me.)
