Nell
Nell is a metaphor for all that is good and right in the world. It is not unusual to see young girls being used this way in literature, and it is clear that Dickens is employing the same mechanism to display the innocence of youth and the sacrifices made by those that are innocent, as displayed by Nell’s eventual death.
Quilp
In direct contrast to Nell’s role, Quilp's is a metaphor for all that is wrong in the world. His very appearance is described in unpleasant terms and his physical deformity only adds to his monstrous personality. The fact that Quilp further adds fuel to the fire by encouraging Fredrick to track his father, highlights his sadistic nature.
Old Curiosity Shop
The shop is a metaphor for a sort of utopia – it is a safe haven for Kit, Nell and her grandfather. Once they are cast out of the place, they are faced with the harsh reality of life, which presents more like a dystopia.