Dinah Brand is a femme fatale for sure, but she also manages to break that mold.
What's a femme fatale, you ask? It's a French term meaning deadly woman. A femme fatale is attractive, aggressive and sexually provocative. She uses her wily charms to lure men into dangerous situations to serve her own interests.
In many ways, Dinah does seem to fall under the category of the typical femme fatale, especially since she knows how to control her men using her femininity. But Hammett also introduces some interesting modifications to his portrayal of the seductive Dinah. Take a look, for example, at how she's first described by the Op. Dinah appears to be a little overweight, sloppily dressed, with bloodshot eyes:
Her course hair – brown – needed trimming and was parted crookedly. One side of her upper lip had been rouged higher than the other. Her dress was of a particularly unbecoming wine color, and it gaped here and there down one side, where she had neglected to snap the fasteners or they had popped open. There was a run down the front of her left stockings.
This was the Dinah brand who took her pick of Poisonville's men, according to what I had been told. (4.14)
This description isn't exactly what we'd expect to be the alluring but deadly vixen of the novel. Even though Dinah isn't the most stunningly beautiful femme fatale out there, the Op does describe her hand as being "soft, warm, strong" (4.13), all positive attributes and the Op can't deny that she does have a mysterious "It" factor that makes her attractive to men.
And the more we see of Dinah, the more she grows on us. Her obsession with money would normally come off as petty and materialistic, but she is so upfront and honest about her mercenary interests, that we find it funny and even endearing. Dinah also sees money not merely so that she can buy things she wants, but because it gives her a sense of control and stability.
As Dinah and the Op send more time together, even the Op starts to let down his walls and confide his fears to Dinah. That's why her death comes as such a shock in the novel. Out of all the corpses that start to pile up, Dinah's death is presented as the novel's most critical moment. And the Op's emotionless reaction to her murder is possibly the most morally troubling scene in the novel.