How does the author use foils to reveal Jane's character in Jane Eyre?
There is a sense in which that in this novel, which is so dominated by its protagonist, the other characters only exist in terms of what they show about Jane's character. Throughout the novel one of the key conflicts that Jane endures is the battle between reason and emotion, or intellect and passion. We see this in the famous red room incident, when she raves like a "wild cat," and gives in to her passion. However, we also see the cold, detached way in which she acts towards the end of her time at Lowood and at the beginning of her time at Thornfield. What is interesting is the way that the majority of the characters in the novel represent one or other of these two unhealthy extremes, thus acting as foils to Jane. Thus it is that Helen Burns, through her stoicism, acts as a foil highlighting Jane's passion and emotion. In the same way, Rochester, with his passions highlights the way that Jane at this stage in the novel is actually dominated by reason.
An interesting example of this comes when Jane returns to Gateshead for the death of Mrs. Reed and spends time with her cousins, Georgiana and Eliza. Note how she describes the differences between them:
True, generous feeling is made small account of by some; but here were two natures rendered, the one intolerably acrid, the other despicably savourless for the want of it. Feeling without judgement is a washy draught indeed; but judgement untempered by feeling is too bitter and husky a morsel for human deglutition.
Both Georgiana and Eliza then represent the two extremes that Jane has to battle between in her life, and at this stage in the novel, when Jane seems to be somewhere in the middle of the spectrum, they represent the dangers of either one, reflecting on Jane's past conduct and also commenting on the unresolved tension within her. It is only at the end of the novel that she is able to keep the two in a happy tension inside of her and live a life that is not dominated by either her passions or cold intellect.