As sisters with extremely different personalities (who are pursued, at one point, by the same man), Dorothea and Celia act as foils for each other. While Celia is focused on fulfilling traditional gender roles (by becoming a housewife and mother who leaves politics and intellectualism to the men), Dorothea is deeply interested in challenging what it means to be a woman in her particular society.
In the very first page of the novel, the narrator comments on the differences between the two sisters:
[Dorothea] was usually spoken of as being remarkably clever, but with the addition that her sister Celia had more common-sense.
Celia’s “common-sense” comes through in the way that she marries strategically and doesn’t get caught up in ambitious goals. Dorothea, on the other hand, is cleverer and more interested in affairs outside of the typical woman’s domain—this is apparent in her commitment to designing cottages for her uncle’s tenants, her desire to marry Casaubon so as to support his important scholarship, and her support of Will’s position running a progressive newspaper.
While both sisters end up married and having children with men they love, Celia ends up satisfied and Dorothea does not. In a way, Middlemarch is Eliot’s somewhat pessimistic portrayal of how women who resist gender norms may end up less happy than those who do not.
As Dorothea’s two love interests, Casaubon and Will act as foils for each other. Will is young, romantic, and idealistic—like Dorothea, he cares about progressive politics and wants to make a difference in the world. Casaubon, on the other hand, is old, stuck in his ways, and focused on his esoteric book—The Key to All Mythologies—that, ultimately, he is unable to finish before his death.
Like Dorothea, Will is external-facing—he wants to engage deeply with the world—while Casaubon is internal-facing—focusing on his studies and quite literally going to and from the library every day during their honeymoon in Rome. Will reflects on this quality of Casaubon’s after running into Dorothea in Rome, highlighting the differences between the two men:
[T]he idea of this dried-up pedant, this elaborator of small explanations about as important as the surplus stock of false antiquities kept in a vendor's back chamber, having first got this adorable young creature to marry him, and then passing his honeymoon away from her, groping after his mouldy futilities (Will was given to hyperbole)—this sudden picture stirred him with a sort of comic disgust: he was divided between the impulse to laugh aloud and the equally unseasonable impulse to burst into scornful invective.
Will’s description of Casaubon as a “dried-up pedant […] groping after his mouldy futilites” who fills him with disgust highlights all that Will does not want to be. Additionally, Will’s impulse to use hyperbole in his characterization of Casaubon (as the narrator notes) also captures something important about Will’s character as opposed to Casaubon’s—he has a certain intensity and vitality that Casaubon does not.
