LT, was Valis' creature your she-devil? I'd understand your description except the term "horrific"...
More seriously now, you wrote:
From a biblical perspective, I understand that Satan is refered to as a "he" (but then, so is the Holy Spirit, whom I suspect is a "she"). However I don't see anywhere that has Satan as the absolute opposite of "God". Yes he is characterised as evil, but not "absolute evil" as a counterpart to "God's goodness".
I agree that there is no archetypal opposer of God in the OT, but in the N.T., and especially in John, I think this is exactly what is meant by "the devil". He is the "ruler of this world", and is radically and eternally evil (with the same relation to God as the darkness has with the light in the Prologue):
He was a murderer from the beginning and does not stand in the truth, because there is no truth in him. When he lies, he speaks according to his own nature, for he is the liar and his father. (8:44.)