In Chapter 1 of The Brief Wondrous Life of Oscar Wao, Díaz uses a character foil to contrast Maritza and Olga, Oscar’s two young “girlfriends.” When he was seven years old, Oscar had two girlfriends at the same time. Maritza was beautiful, glamorous, and rich. She was friends with Lola. Olga was the exact opposite: she was poor, seen as ugly, and Puerto Rican, which, in Dominican culture, meant that she was at the bottom of society.
These two girls are present in the novel insofar as they are jokingly referred to as Oscar’s “peak” —the last time he had any success with women. The stark foil relationship between them contributes to the sense that they are symbols rather than people. They serve to sharpen Oscar's character, providing a humorous story that illustrates his lack of romantic success since then. But they serve more to demonstrate a certain thing about Oscar than they do to be characters in their own right.
This is somewhat representative of the role of women throughout the novel. Until Ybón, Oscar does not have fulfilling relationships with women. As such, the female characters beyond his family members enter the novel to reinforce, contradict, or sharpen parts of Oscar's life and personality.