Patrick Bateman does not exist physically but rather is the narrative occupying the minds of all the characters we see and this is eluded to numerous times. The opening dialog itself states "I simply am not there" which was timed so it's said when he peels off the mask. The entire dialog once the mask has dried eludes to his lack of physical existence and being the "spirit" that occupies all of these people.
He's talking about the emptiness inside these people he represents and the elitist vanities they use to try to fill them and justify their despicable personalities. A group doppelganger or tulpa but he simply doesn't exist physically. When his secretary says "thanks Patrick" he says "I'm not here".
Even the friends of Bateman humiliate him and in the beginning they don't even know which man is Paul Allen. When we see this person, whoever he is, he is drinking himself into an early grave.
Notice the head is there as a projection?
It could actually be anybody sitting there but it's The Bateman driving him to sleep with his coworker's fiancée.
Bateman telling people what to do, what he thinks, the standards he sets etc. is the inner voice driving all of these people and sometimes it's internal and other times it's external but we see him as Davis who really exists or Allen who really exists when they are all being The Bateman.
He is the embodiment of everything these people strive to please and be. You notice in conversations everyone centers around him, praises him, seek his approval?
Every emotion he describes is a reflection of every character in the movie right down to the murders but they leave out the "if I could get away with it". There's copious amounts of evidence validating what I am saying. There's at least one example in every scene that verifies what I'm saying.
Not only is there no Patrick Bateman but the physical character we are following doesn't exist. If you rewatch it a few times in a row you can see Patrick Bateman's physical existence represents many things not just thoughts or actual people. Sometimes someone shows up and we see an opportunity to accept this theory as the Bateman unleashed was in fact Victoria's inner Bateman.
We are seeing The Bateman inside of everyone.
We are watching a movie about all the secondary characters who are in fact the leads but only when they are being The Bateman.
Of course the one last concept is indeed Bateman exists as a Phantom and passes from body to body.
It's as close to art as film making can ever get and one of the few films out of hundreds of thousands that deserves to be called art.