Biddy is some complicated relation to Mr. Wopsle, and she's also a glimpse into what Pip's life could have been if he weren't such a butthead. Although at first she's a messy little girl who always wants "brushing … washing … and mending" (7.4), she grows up into a helpful, intelligent, and kind woman:
Imperceptibly I became conscious of a change in Biddy, however. Her shoes came up at the heel, her hair grew bright and neat, her hands were always clean. She was not beautiful,—she was common, and could not be like Estella,—but she was pleasant and wholesome and sweet-tempered. She had not been with us more than a year (I remember her being newly out of mourning at the time it struck me), when I observed to myself one evening that she had curiously thoughtful and attentive eyes; eyes that were very pretty and very good. (17.3)
The whole point of this description is the Biddy is basically the anti-Estella: common, but kind, pleasant, sweet, thoughtful, and attentive. (To be honest, she's a little too good to be true.)
She also grows up to be in love with Pip—but he only notices her long enough to say that he wishes he could just force himself to fall in love with her already. The one thing we can't quite understand is why she ends up marrying Joe at the end of the novel. Does she see marrying Joe as a way of getting closer to Pip? Could she possibly have fallen in love with an illiterate man who's goodness knows how many years older than she is?