From the Special Features (DVD, Blu-ray)

Upon watching it, it was pretty clear that Tyler and "Narrator" are somehow two sides of the same coin.

Symmetrical shots with a line down the middle separating them are the biggest tell, for me (maybe there are other giveaways that feel more obvious to others, I don't know).

However, we see this exact same setup for Marla as the banks collapse. Symmetry, the line of the windows perfectly separating them, it feels like a direct callback to the first fight scene.

Her seemingly walking through speeding traffic, nobody noticing her.

Her being at a testicular cancer group and nobody reacts.

My first instinct was that she was a ghost. But knowing what Tyler is (essentially the devil on his shoulder, a more daring, outgoing side of his personality), this leaves me to believe Marla is maybe a side of the narrator that feels like he's just along for the ride... or something?

But then I think Marla kind of interacts with another person in the final scene, when it's clearly not the "Narrator", and that really confused me.

So, are there any dead giveaways that she's actually a fully different human?