This is a very interesting question to consider, because it is possible to view every other character in this book as being relevant only in the way that they cast light on the central character, that of Jane Eyre. Jane is juxtaposed and influenced with almost every character in the book, and Adele is no exception. You might like to consider the way in which Adele and Jane are presented as doubles of each other. Both are orphans, and both find themselves dependent upon others. Note, for example, how Jane in Chapter Fifteen responds to being told about Adele's past and that she is the illegitimate offspring of a French opera singer who was wanton in her sexual relationships:
Adele is not answerable for either her mother's faults or yours; I have a regard for her, and now that I know she is, in a sense, parentless--forsaken by her mother and disowned by you, sir--I shall cling closer to her than before. How could I possibly prefer the spoilt pet of a wealthy family, who would hate her governess as a nuisance, to a lonely little orphan, who leans towards her as a friend?
Jane's personal experience of life as an orphan and being disowned by everybody gives her a real appreciation of Adele's position. In addition, you might like to think about the way that Adele, like so many other characters, represents one extreme that Jane has to battle against: that of being overpowered by passion and sensibility. Adele is persented as rather a silly little girl who is overly fond of presents and dresses. The way that her feelings rule her is something that Jane recognises and works to correct. Yet Jane herself has been subject to the overwhelming force of her feelings: let us not forget the famous Red Room incident. In this way, Adele presents another aspect of Jane's internal psyche.