Bertha Mason, as presented in the book Jane Eyre, is everything Jane is not. Jane is depicted as being physically plain and slight of build. She is highly intelligent, and, her tendency to be outspoken notwithstanding, she is a proper Englishwoman, comparatively cultured, fine, and neat in appearance and manner. Jane is capable, and sensible. She is a survivor, is passionate, as well as strong in character. In contrast, Bertha is monstrous, barely human. Although she shows with her sinister actions that she is capable of some awareness and rational thought, recognizing Jane as as a threat to her position and seeking her demise to prevent that from happening, she cannot speak intelligibly. Bertha is presented as being exotic and unbridled; she is violent, clearly insane, and exhibits an animal nature.
Despite the clear differences between them, however, it has been suggested by some critics that Bertha's character runs parallel to that of Jane's, with Bertha being a kind of "darker double of her English counterpart. In support of this theory, it is interesting to note that Bertha lives confined to the attic at Thornfield, just as Jane was earlier locked up in the red room at Gateshead Hall. Both women are or have been a source of attraction to Edward Rochester, and both are at one point married to him.