6. Jack & Marla
"All of this, the gun, the bombs, the revolution, has got something to do with a girl named Marla Singer."
Like Tyler Durden, Marla exists in Jack's mind as a reaction to the world around him, namely a society that increasingly devalues traditional (and often toxic) traits of masculinity.
There are countless pieces of evidence, admittedly circumstantial, which point to Marla's non-existence, both through her interactions with Jack and the world around her.
Let's start out simple - she shows up to lung cancer support groups while smoking cigarettes and nobody says a thing to her. That's pretty weird, but in of itself excusable, right?
If we look to how she relates to Jack, the picture becomes clearer. Marla starts finishing Jack's sentences shortly after they're first acquainted, in a similar fashion to how Tyler understands Jack's self-satisfied "single-serving friend" shtick immediately, because they're sharing the same brain.
Furthermore, Marla says that she saw Jack practising telling her off for attending the meetings, which is quite clearly a sly nod towards the central twist of the movie - and Marla simply being another part of it.
Hilariously, when they're divvying up their support groups, he tells her, "you can't have the whole brain", which again feels like a playful nod to the fight between Marla and Tyler for control of Jack's psyche.
In more overt physical terms, Jack watches Marla walk into traffic with the cars passing by her as though she isn't even there - and that's because she isn't. Jack is effectively having an argument with himself on the sidewalk, and just as he projects scenes that aren't real with Tyler, he's doing the same with Marla.
Marla also sells some clothes and is curiously seen taking several pairs of what are clearly men's jeans. You could argue that she stole the jeans, being poor and all, but could it not also point to the fact that these are, in fact, Jack's jeans?
There's another clothing link later, when Marla talks about the bridesmaid's dress she bought from a thrift store for $1, telling Jack that he "can borrow it sometime", pointing to not only their shared body but the possibility that Jack might enjoy dressing up as a woman.
Elsewhere, they both talk about being "on their way out" when Marla makes a suicide attempt, and when Tyler appears to rescue her, she talks about herself in the third person as "infectious human waste", which immediately conjures up thoughts of the later scene where Jack and Tyler steal fat from the liposuction clinic (which has an "infectious waste" sign visible).
The timeline doesn't sync up, but again, it's not entirely discounted because we're seeing a distorted, unreliable version - and timeline - of events.
And finally, deep in the movie's third act, Marla makes the off-hand comment to Jack that the Project Mayhem goons were burning their fingertips with lye.
Now, it's fair to say that the average person has no idea what lye looks like, especially as it's a totally generic, powdery white substance, and so for Marla to have such a specific piece of knowledge again points to her sharing a brain with both Jack and Tyler.