After the end of the movie, you realize that The Girl is the Devil and has chosen Corso for the gift of the power since the beginning.
Alternate interpretation: Corso is actually Satan and has undergone a Memory Gambit to return to heaven. The book is supposed to "raise the devil" and so it does, by showing him the way back home.
Not only that, but when Balkan tries to summon the Devil, who appears but Corso, who even says "I'm the only devil you'll see tonight." Sure, Balkan's plan was flawed due to the forgery of the ninth engraving, but the symbolism is there.
In the book The Girl/Irene Adler is not the Devil, but still a fallen angel, and Corso is a normal human haunted by tragedy in his life and his ancestor's. In the film, either character can be seen as the Devil. Kind of like how the three books each hold some answers.
There's another possible interpretation of the film (though not necessarily the book), in that the Ninth Gate really had nothing to do with Satan. According to this theory, the "Lucifer" referred to as the author of the Delomelanicon referred to the name's literal translation of "Lightbringer," and not the demon of Christian mythology. The engravings serve as a map of consciousness, not unlike the Kabbalah, and lead down two separate paths, one a path of greed and self destruction which ends in flames, and the other, a true path to enlightenment. Aristide Torchia (and by extension, all those who interpret the material as Satanic) follows the negative path and dies as a result. Dean Corso somehow ends up on the right path, and achieves transcendence. A much more detailed explanation can be found here
Fridge Logic: Considering Corso and the other characters are all experts on rare books, why does Corso never wear any sort of hand protection when examining them and is allowed to drink and smoke near them? The Baroness is the only one who seems to be annoyed by such behavior, rightfully so.
The books being laid out on the floor in Fargas' mansion was actually taken from the source novel. Not that it necessarily makes it better....
Books during the era were printed on extremely heavy paper or vellum, not mere pulp, with bone spines (hence the name "spine") and leather covers, smoking and drinking near them is probably the lightest they were treated in their entire life.
Truth in Television: Real old books are commonly handled without any gloves (though smoking and drinking around them are definitely not allowed), because wearing gloves leads to loss of tactile function, which could actually lead to inadvertently tearing pages or damaging the book. Furthermore, if a book happens to get damaged, it's often repairable.
Why are they speaking Spanish in Portugal, anyway?
When did they? The only Spanish was being spoken by the Spanish Ceneza brothers in Toledo, Spain, to my recollection at least. I don't recall any Spanish during the short time Corso was in Portugal seeing Vargas.
The Ninth Gate had everyone I know making that noise Scooby-Doo makes when he's confused. And sometime later, the ending hit me: The girl was Satan herself, and Korso had found the ninth gate and used it. —Indigo
She is very unambiguously Satan in the book on which the film is based, El Club Dumas by Arturo Perez-Reverte.
She's a fallen angel, but not Satan, in the book.
What I got from the film was that Korso was an amnesiac Satan and the girl was a witch or demon who was trying to guide him to the ninth gate. —Vashthe Stampede
Some fan discussions point to the final engraving as showing the Whore of Babylon riding on the 7-headed beast out in front of a flaming castle, so they think The Girl is her.
My dad loved the movie, then one evening randomly invited me to watch it with him. He'd seen it several times, but had never noticed that the last carving/illustration Korso discussed or examined was foreshadowing. (example- Korso examines the carving that resembles the Tarot card The Hanged Man, and then goes back to his shop to find his friend dead and dangling from his ankle, in the same pose as the picture.) I thought it was pretty obvious, but then again, my dad is kind of thick when it comes to anything that isn't math-based. -Timber!
What really made this sink in for me is how Balkan says in the beginning how using the proper text will cause the Prince of Darkness to appear in person. As soon as Balkan puts the pages in the correct order at the castle he looks up... and Korso is standing right there.
But the book wasn't complete at that time. Balkan had eight genuine engravings and the false one created by the Spanish brothers.
I assumed the girl would be satan from the start knowing Roman Polanski had cast his own wife and his general obsession with making movies about the devil. Read the book later, it's far more obvious in the book than the film.—bluebrainfreezy