This woman is the reason and the main figurant of the story’s suspense. She is precisely that secret which Rochester tried to hide. She is that secret wife. When she was younger, she used to be very beautiful, wealthy, smart and, mainly, a desire of all men. But through some years she became insane, violent and bestial. Rochester locked her in the secret room on the third floor after she tried to kill him. Grace Pool was the woman who was guarding her, but one day she failed, Bertha escaped and burned down Thornfield dying there and making severe injuries to some people.
Bertha Mason in the Essays