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.
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:
Unlock with LitCharts A+[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.