I am studying Jane Eyre as part of A-Level English Lit, and thought up an interesting theory. I won't go into it fully on here, as it would be unfair to copy and paste a massive chunk of writing for you on here, but I will provide a link to my blog which goes into it in detail.
Basically, what if Jane Eyre IS Bertha Mason? Not in a metaphorical sense of 'Oh, Bertha is an embodiment of Jane's passion' or 'Oh, Bertha is just a crazy Jane', but what if Jane Eyre is a persona created by Bertha Mason to escape from her imprisonment in Thornfield? The whole of Jane's life is a way of Bertha doing everything she couldn't in reality, and so Jane is not real, and the whole of Jane Eyre is a life thought up by Bertha in her imagination as a way to cope whilst she is stuck, insane, in a room in Thornfield?
It would be great if you could all go to my website and read the full theory, it explains a lot I haven't covered here. This only partially a shameless plug
Loughborough at number one