Who is Kate Cutrer from The Moviegoer and what is their importance?

From the beginning of the novel, it is clear that Kate is an important figure in John's life. When he meets other family members and friends on the street, they ask about Kate. Kate is John's cousin, but she shares a unique and special bond with him. She suffers from serious depression, but feels her best when she is around John. She accuses of being "worse" than she is simply because while others know and acknowledge Kate's depression and fragile state, they only see the good and quiet aspects of John, ignoring that they are very similar people.