In Chapter 15, where Rochester tells Jane about his past affair with Adele's mother, and how he came to be Adele's guardian, Rochester admits to Jane that he and Celine Varens were lovers and that he had what he called a "grande passion" for her. What is key to realise from the description of Celine Varens is that she seems to be a character who knows no restraint in the expression of her passion and desire, as, in response to Rochesters "grande passion," she evidently returned his lust in a more intense fashion:
This passion Celine had professed to return with even superior ardour. He thought herself his idol, ugly as he was...
Of course, Rochester discovers that Celine had relationships outside of her affair with him, and it is this discovery that causes him to end his affair with her. Celine is therefore presented as yet another figure in this novel who is unable to restrain her base desires and instincts, in the same way that Mrs Rochester is unable to, and even, to a certain extent, Rochester himself, as is shown when he desires Jane and wants to marry her even though that would be against the law.
By contrast, Jane is a foil to Celine Varens as she is a figure who restrains her passions and desires. When she has the opportunity to go to Europe and live with Rochester as his mistress, away from the gaze of society, she refuses, as she does not allow her passions and desires to overpower her rational intellect and reason.