Kismine Washington is Percy's youngest sister and the love interest for our hero. She is, like her father's diamond, flawless. Let's take a look at the blossoming of first love:
[John] was critical about women. A single defect—a thick ankle, a hoarse voice, a glass eye—was enough to make him utterly indifferent. And here for the first time in his life he was beside a girl who seemed to him the incarnation of physical perfection. (5.11)
Kismine is an extension of the seemingly flawless, beautiful Washington château. But like the prison that lies beneath the ground of the estate, a darker secret lies underneath. (Could that sound any more ominous?) Kismine is just using John for the summer – he'll be killed at the end. She knows this, and yet does nothing to stop it. It upsets her, but only in the way having a headache on your birthday would make you upset. She doesn't see anything fundamentally or morally wrong with the picture – she's only depressed that thinking about the matter will take all the fun out of her summer. "It's only natural for us to get all the pleasure out of them that we can first," she tells John (8.33). She claims that she's "honestly sorry" about the whole mess, though admits that she would rather see John "put away than ever kiss another girl" (8.41).
The problem here is that Kismine doesn't understand the value of human life – or death. "We can't let such an inevitable thing as death stand in the way of enjoying life while we have it," she explains to John. "Think how lonesome it'd be out here if we never had any one. Why, father and mother have sacrificed some of their best friends just as we have" (8.39). This problem of valuation continues throughout the story. Later, when the slaves quarters' are destroyed in the bombing, she laments, "There go fifty thousand dollars' worth of slaves, at prewar prices. So few Americans have any respect for property" (9.24). Kismine's inability to understand the real value of anything – not just its monetary value – is the result to her upbringing. She's used to being able to buy anything – and we mean anything – so it's no surprise that she has little respect for non-monetary value.
Just as her father is made the victim of his own shortcomings, so is Kismine at the end of "The Diamond as Big as the Ritz," when she brings rhinestones in her pockets instead of diamonds. "I think I like these better," she says. "I'm a little tired of diamonds" (11.11). Kismine is indeed tired of her life of luxury and ease, demonstrating once again that money can't buy happiness. She's bored with her life to the point where she's excited about the prospect of being poor. "'We'll be poor, won't we?' she says to John with childish delight, 'free and poor. What fun!'" (9.28).