As Gilbert’s letter to his brother-in-law Jack Halford opens and closes the novel—with Helen’s story in her own words in between—it can be considered a frame story. This is because Gilbert is the person who introduces readers to Helen and “frames” the story via his own descriptions and opinions of her.
This is a noteworthy choice on Brontë’s part given her thematic focus on the ways in which men have more power than women within her society. Brontë likely gives Gilbert the power to frame Helen’s story as a nod to how he has more power in their marriage and, in public, is allowed to speak for both of them. Still, that Brontë allows Helen to “speak” for herself via her diary entries shows that she is committed to challenging Gilbert’s authority.
It is worth noting that Gilbert is an inherently unreliable narrator. The following passage shows how he offers his opinions on Helen’s diary entries and admits to his self-interest in certain parts of her story:
Well Halford, what do you think of all this? […] I will only make this acknowledgement, little honourable as it may be to human nature, and especially to myself: – that the former half of the narrative was, to me, more painful than the latter, not that I was at all insensible to Mrs Huntingdon’s wrongs or unmoved by her sufferings, but, I must confess, I felt a kind of selfish gratification in watching her husband’s gradual decline in her good graces, and seeing how completely he extinguished all her affection at last.
Here Gilbert admits to Jack that he “felt a kind of selfish gratification” in reading about Helen’s misery in her abusive relationship with Arthur because it meant that she no longer has feelings for Arthur. In other words, Gilbert is not a neutral and objective narrator.
Gilbert’s own stories about his life—such as when he beats Helen’s brother Frederick with a whip and almost kills him, believing Frederick (incorrectly) to be Helen’s lover—show how he is an occasionally unperceptive and emotionally unstable man. These characteristics also contribute to his unreliability.