Sally Setton, also known as Lady Rosseter, is Mrs. Dalloway's lifelong friend. Ever since she and Clarissa were adolescent, Sally was the foil to Clarissa's character, representing, doing, and acting like everything that Clarissa's much more demure nature would prevent.
As Clarissa's foil, Sally complemented her best friend's personality so well that Clarissa developed a strong connection with her. This is because Sally, in a dramatic contrast to Clarissa, was vibrant, daring, extroverted, and full of life.
Her energy was clearly masculine, as she would instill upon Clarissa a sense of dominance and protection, that resulted in what could be deemed as a platonic love affair between the two young women. However, as females in the male-dominated society of Post WWI England, their duty was to grow up, marry well, and claim a role within the upper-class set.
As older women, Sally and Clarissa continue to be friends although they had not seen each other for a long time before Clarissa's party. Although times have changed Sally and made her less impetuous, abrasive, and idealistic, she is still quite resolved in terms of how and what she thinks. Again, Sally is once more everything that Clarissa would have been had she been a more liberated and less self-constrained woman. Sally is certainly as complementary to Clarissa at an older age as she was when they were younger. Therefore, the role of Sally is to serve as Clarissa's true self; the woman that thinks, acts, and speaks the way in which Clarissa would very much want to have the courage to do.