Marla is the femme fatale love interest in the 1999 thriller Fight Club. Even though she weaves throughout the movie, she is a major part of the story progression.
Marla is introduced in Fight Club at one of Jack's (the Narrator) self help groups. Just like him, she doesn't have any diseases, but finds entertainment in it. Jack and Marla agree to split the groups because Jack can't cry while she's there. Marla makes several more appearances in the film, and furthers the plot whenever she appears. Marla also develops a relationship with Tyler Durden.
The name Marla came from a girl who used to beat up Chuck Palahniuk's sister. When writing the character, he wanted to think of the most despicable name for a woman, so he used Marla.
*Warning Spoilers for Fight Club Ahead*
Marla is established as a core part of the story very early, when Jack has a gun in his mouth and says this all started with a girl named Marla Singer. Author of the original novel Chuck Palahnuik even said that the whole of the story is about a man reaching a point where he can commit to a woman. Whenever Marla appears in Jack's life, his life changes, and is able to confront his love of her more. Jack is not able to confront Marla when she first appears at his support groups. When he does, he stops going to the groups and starts the fight club with Tyler. This represents Marla's effect on his life. When she leaves, Jack needs to use more and more extreme ways of connecting with the world, his problem in the first place. After Jack becomes happy with his life again, Marla appears and begins to have sex with Tyler in his home. She leaves, again, and Jack is able to confront her more.
But in order to connect with the world again, he needs to find another way. This leads to Jack burning his hand with lye (even though he imagines Tyler doing it) and starting Project Mayhem. Project Mayhem is a war on civilization, consumerism and the world they live in. Marla has been responsible for all of this so far. Marla makes one real effort to connect with Jack, but she is denied. He then realizes what he's done and that he is Tyler Durden. Marla makes her final appearance as her and Jack hold hands and watch the buildings Project Mayhem members have blown up because of Marla. She is the root of all of Jack's issues, because he can't admit he loves her because they are so similar.