Satan and The Devil can mean lots of things- it is important to note that all his nicknames mean something different, of course. I definitely agree that The Devil is the Ego, and so does my Catholic priest. That doesn't make The Devil evil- though if you give in to him evil may result- it's all there in the Tarot card. The Devil is the shadow of God, the beast inside us all- the materialism that we must overcome. His pentagram can point in any one of 360 degrees, and that direction, and its interpretation, is up to us. Just because it points down doesn't mean it has to be bad. (Personally I think when it is tilted 17 degrees it is more like Hell. That's just some really, really horrid personal experience that lasted for years.)
Satan has a different flavor and interpretation. The name Satan is, of course, Hebrew, and does indeed mean the prosecuting adversary- I often view it as that he is a critic. He points out the faults so that we may improve ourselves. Satan is also a son of God when he appears.
Lucifer and the serpent are two other names- Lucifer is the lightbearer and is a nickname that applies to Satan, Jesus, Venus, et al. The serpent is a symbolic figure seen in so many ancient cultures and I am sure you can guess some of the symbolism. Apollo killed the Python, after all. It all means something that can relate to our every-day lives. And then of course there is the kundalini- I have often viewed the serpent as such.
Here's how I interpret the story- which I read a bit of in Hebrew but I am not that good at it. Eve (Living One) approaches the Tree of Knowledge of All that is Good and All that is Evil. The divining, subtle serpent (and as I see it, though I don't know that this part is there in the Hebrew, kundalini) tells her that if she eats of it she will become like God. She does. I see this as a representation of when man became self-aware. There is a certain point when mankind became mankind- the first human beings. They became bipedal and got much larger brains, which meant great childbirth pains- but that also means that we can know all that is good and all that is evil; we are no longer dumb animals.
But yeah, The Devil is definitely a symbol for the Ego. He is the downside of Bacchus, too- we get a lot of his symbolism from there. Bacchus can free us, or he can drag us down with his chains. We are always free to throw them off. If we want to. Then, the Ego can become a magnificent tool and friend.
Satan and I have definitely had some good and bad times together. Sometimes we hang out in my dreams. We are brilliant lights with no need of language, just the expression of beauty, and light, and darkness, and interpretation thereof. We are bodies like human bodies, but unlike human bodies, separate and inseparate, eternal, yet local.