Myrtle Wilson's sister Catherine in The Great Gatsbyis a stereotypical flapper. Nick meets her at the apartment party with Myrtle and Tom in Chapter 2. Fitzgerald's description of Catherine's Bohemian style and her conversation with Nick clearly represent the carefree flapper lifestyle of the 20s.
Catherine, despite her apparent frivolousness and laid-back attitude in Chapter 2, is a very loyal sister. After Gatsby's murder and the ensuing inquest, Catherine--when questioned--never mentions her sister's affair with Tom Buchanan. Of course, she might have also had more selfish motivation for keeping quiet. Instead of simply protecting her late sister's name, perhaps she was paid off by the Buchanans as they retreated into their cocoon of wealth, or perhaps she didn't want to be associated with a compromising affair that might hinder her chances to move upward in society. It is difficult to know what her true motivation is. Nonetheless, Fitzgerald does clearly present her as a traditional flapper at the novel's beginning.
In The Great Gatsby, Catherine is introduced to the reader in Chapter Two when Tom and his mistress, Myrtle, hold an impromptu party in New York. Physically, Catherine is described as being "slender" and aged around thirty. With her red hair styled in a bob, she evokes the image of a 1920s flapper girl, which also suggests that she is fashionable and trendy. This is further supported by the accessories she wears: her face is heavily made up, for example, and she wears so many bracelets that she makes an "incessant clicking sound" every time she moves. Though she looks at the apartment as though she owns it, she in fact shares a room in a hotel with one of her female friends, suggesting a strong degree of independence.
That Catherine is image-conscious and fashionable is also shown by her conversation with Nick. It emerges, for instance, that she has attended one of Gatsby's parties and that she is keen to work on Long Island, a very fashionable place. She is also very modern in her attitudes to relationships: she thinks that Tom and Myrtle should divorce their spouses since they clearly cannot stand to be married.
Catherine, therefore, symbolizes the carefree and modern flapper girl of the 1920s who is staunchly independent and keen to make her mark on the world.
It should also be noted that it is Catherine that is used by the author to let us know more about Tom and Myrtle's relationship. It would have seemed odd coming from Myrtle or Tom directly. She plays a significant role in allowing us to understand Tom's feelings about Myrtle and how he is still trying to maintain a relationship with Daisy.