The witch is the devil....
I wouldn't go that far. A "Satan" figure was introduced in later books. Jadis (the Witch) began life as a mortal, albeit an unusually powerful one, so she isn't really comparable to Satan. She's a representation of evil, for sure, though.
As far as the books go, Jadis is in fact older than the Universe Narnia inhabits. She came from another world, brought to Narnia by humans who had used magical rings to breach the barriers between alternate Universes.
In the book of
The Lion, the Witch & The Wardrobe, the Beavers say that she's a descendant of Lilith; reported by some sources to have been Adam's first wife (before Eve). According to these sources Lilith was created from dust as Adam was, while Eve was created from Adam's rib. When Lilith refused to assume a role subservient to Adam, she willingly left the Garden of Eden. As such she was in a unique position among humanity; as one who left Eden of her own volition, and remained unaffected by the fruit of the Tree of Knowledge of Good and Evil. In the Narnian continuity, perhaps she not only left Eden, but left this Universe completely, and took up residence in Charn (the world Jadis originated from).
She seems to have begun as a Mesopotamian night demon, and appears as a night demon in the Midrash and Talmud. She translates as a "screech owl", for what that's worth, in the King James version of the Bible (as always, english translations of Christian documents prove useless in terms of real study).