What is the Devil?

Inspired by ETpro’s question

I’m looking for answers by both Christians and Atheists. (and people of other beliefs, or lack thereof, that does not relate to those two)
Who is Satan? What does he do, what does he want, what’s his problem? How do you see him as? Does he look like anything? Is he really some being somewhere, using deceit and evil to harvest souls? Is he a metaphor for societies, past and present?
If he’s not real, why have we created him? If he is real, what can you tell me about him? Is he some freaky goatman, or everyone’s worst fears? What is Hell?
The Devil; what is he to you? I am indeed an atheist, but I’m seriously looking for serious answers, and this is not meant to offend, whatsoever. I know religious stuff can get messy around here, and I don’t want this to happen in this question. I’m really wanting to know what the Devil is to everyone.
As well, if anyone is knowledgeable in mythology, history and religious oriented elements about Satan, that would be cool, too.