The modern-day equivalent of Carmen Sternwood would probably be someone like Paris Hilton. A beautiful, rich, spoiled socialite who's flirtatious, self-absorbed, and probably a whole lot of trouble. Carmen is the younger of the two Sternwood sisters, and she makes a pretty memorable entrance in the first chapter of the novel:
She was twenty or so, small and delicately put together, but she looked durable. […] She walked as if she were floating. […] Her eyes were slate-gray, and had almost no expression when they looked at me. She came over near me and smiled with her mouth and she had little sharp predatory teeth, as white as fresh orange pith and as shiny as porcelain. They glistened between her thin too taut lips. Her face lacked color and didn't look too healthy. (1.6)
At first Carmen appears to be a delicate, innocent creature who couldn't hurt a fly. But as we read on, there's something unsettling about her appearance. Her blank stare is straight up creepy and her "sharp predatory teeth" make her sound like a vampire. And it doesn't help that her face lacks color either. This dame sounds downright cold and bloodthirsty.
A few paragraphs later, Marlowe gives us a strange sexualized description of Carmen sucking her thumb: she "bit it and sucked it slowly, turning it around in her mouth like a baby with a comforter" (1.21). So Carmen appears to be both childlike (notice the image of the "baby with a comforter") and sexually provocative in the way she bites her thumb in front of Marlowe, which, it turns out, is the perfect mixture to sum up her behavior for most of the novel: innocent ingénue one minute, and shameless vixen the next.
If Marlowe plays the role of the knight errant (See more in Marlowe's "Character" section), then that makes Carmen the first damsel in distress Marlowe encounters on his journey. Marlowe sees Carmen naked not once, but twice. First, at the scene of Geiger's murder, a drugged Carmen appears naked just like the damsel in the stained glass. And second, a sober Carmen shows up in Marlowe's bed without a single shred of clothing on her body. And of course, she plays the "I'm innocent" card. She sucks her thumb and tells Marlowe over and over how cute he is. When Marlowe rejects her advances, the trouble starts:
Then I was aware of the hissing noise very sudden and sharp. It startled me into looking at her again. She sat there naked, propped on her hands, her mouth open a little, her face like a scraped bone. The hissing noise came tearing out of her mouth as if she had nothing to do with it. There was something behind her eyes, blank as they were, that I had never seen in a woman's eyes. (24.36)
This description should probably make the hair on the back of your neck stand up. Talk about creepy. Carmen sounds like a cross between a poisonous rattle snake and a ferocious tiger about to pounce on her prey. The sudden shift from child-like innocence to dangerous predator makes Marlowe question her whole damsel in distress act. And wisely so. She's clearly not a helpless female in need of rescuing. She's just another femme fatale, emphasis on the fatale.
In the final pages of the novel, we finally find out that Carmen isn't as innocent as she looks when she takes a shot at Marlowe while he's teaching her how to use a gun. Vivian confesses to Marlowe that her sister murdered Rusty Regan, in much the same way that she tried to kill Marlowe: "She came home and told me about it, just like a child. She's not normal. I knew the police would get it all out of her. In a little while she would even brag about it" (32.61).
A fair few literary critics believe that Carmen suffered from epilepsy (which would explain the hissing and seizures). And Vivian seems to be making excuses for Carmen by saying that she's not "normal." But does her condition make her less morally responsible for her actions? Is Carmen unable to control her own actions or did she think she could get away with murder? In the morally corrupt world of 1930s L.A., a damsel in distress turns out to be the murderess, and we're left wondering along with Marlowe if there's anyone left to save.