The Bell Jar
As Esther's friend, Joan also happens to be Esther's foil because their lives are so similar…yet different. Although both of them attempt suicide, and both of them end up at the same institution, Joan ends up committing suicide, while Esther goes on to her exit interview, ready to enter the world at large. Joan is in the novel to make us question what difference between the two led one to commit suicide and one not, and, since Esther is the main character, what qualities Esther has that prevents her from attempting suicide again. You could say that Joan makes Esther look a little more stable because Joan is an example of someone who couldn't escape the "bell jar" of depression.