It is here that we finally meet Bertha Mason. It is not quite clear at first who she was, with her presence through the strange occurrences in Thornfield Hall dismissed by Rochester as caused by one of his staff, Grace Poole, instead. It is only during the wedding ceremony between Rochester and Eyre, that the truth emerges: Rochester had already been, and still was married, to Bertha Mason, a wealthy lady of Creole heritage.
I can't begin to imagine how deeply distressing it must have been growing up for Bertha. Creoles are born of mixed heritage, from white European settlers and West Africans. Thanks to this, Creoles are often left feeling non-belonging. To their White European superiors, they were seen as more worthy than the African slaves, but not as equals due to their mixed heritage. Among their African contemporaries, they were seen as traitors for owning African slaves and assimilating into the white European society. To be half Creole, and half White, Berth must have struggled with her own identity of culture and heritage.
It is also through the eyes of both Eyre and Rochester that we learn who Bertha Mason was. In the entire novel, Bertha was only heard through her maniacal laughter or guttural growling. She never spoke for herself, even when we meet her in person. Rochester immediately declared her insane; he blamed her heritage for her insanity.