WG: Where did he come from? Here's my definitive answer based on 50 years of scholarship:
Idunno.
The Lucifer passage - which is really Latin and was never a name for anybody or anything in Isaiah's time - is about the king of Babylon, not a supernatural being. It even says it right in the context, yet we've abused it for millennia.
Ditto the Ezekiel passage. Not about Satan. The only mentions in the Hebrew Bible are in Job.
Does that make him a myth? Not really. But it does suggest that we who acknowledge his/her/its existence have been asking the wrong questions and barking up the wrong trees.