Sally Seton, Lady Rosseter, plays a big role in Clarissa's life. A childhood friend of the now Mrs. Dalloway, Sally is a product of the working classes. Once much lower in social rank than Clarissa, Sally was red-blooded, rowdy, rebellious, and quite idealistic for a girl who is meant to be "down-to-earth" in comparison to Clarissa.
In Sally, Clarissa was able to free her real thoughts and emotions. Sally was her leader in free thought, free speech, and free ideas. Together they had planned many things for the future. There is also a strong indication that Sally's masculine energy played a role in Clarissa's developing sexuality, as the two had a Platonic, yet, sexually tense friendship.
Now Lady Rosseter, Sally is basically living like the people she once judged as a young rebel. However, she maintains her working class sense of fairness and analysis. She does not at all fulfill the role of the martyred wife and mother that Clarissa seems to exemplify and this clearly might be another eye opener for the latter.
In all, Sally is a foil of Clarissa; the childhood friend who vows to break with every established paradigm and social construct of the day. She is in one way Clarissa's philosophical liberator.