Jenny Wren is the alias of a girl named Fanny Cleaver from Charles Dickens's Our Mutual Friend.
It is unclear how old she is, and, according to the book, hard to guess, due to her bad figure, or as she says, "back's bad and my legs are queer!"
She appears to be quite young (about twelve), but has a kind of cynical wisdom that is beyond her years.
She is the daughter of a drunkard, known to Jenny as her "bad boy"--he relies so much upon her for care, that he acts more like a father than a child, and a bad child at that. She has, in fact, almost convinced herself that she is actually his mother.
She makes her living as a dolls' dressmaker, which she has apparently been doing for years, and is quite good at.
Jenny seems to live in her own world, which is a cross between a harsh reality and a poetic fancy. Because of this, and her 'bad child', she often comes off as odd, creepy or eccentric. She has a much softer side, though, which comes out when she's with the few friends she has; Lizzie, Mr Riah (who she has nicknamed "Fairy Godmother"), and, at the end, Mr Sloppy.
It is left to the reader's imagination whether she ends up marrying Sloppy.