Who is the nun?
Well, I'm not sure exactly what you mean, but let me answer the best I can.
The Nun is one of many characters on an anuual religios pilgramage from London to Canterbury. More specifically, to Canterbury, which is a city in England famous because of Canterbury Cathederal, where the Arch-Bishop of Canterbury (head of the Church of England) resides.
Each of the people tell a tale to pass the time, hence the name of the work.
In the Prologue we learn that the she is a French Prioress (or nun) of the Catholic faith. She is the epitome of manners and cleanliness. She is described as being very charitable and very sensitive. She weeps over any dead thing. She carries a rosary. Additionally, we see that she is a bit vain and where's expensive jewelery and clothing. Pehaps she is not as devoted to her work as it might first seem.
In her tale she starts with a long prayer to the Virgen Mary and tells a story of a devoted boy who loves the Virgen Mary and learns to sing a difficult Latin song in her honor. A group of "wicked" Jewish assains slit his throat for this. In the end the Catholics honor his memory and "the wicked Jews are tortured and killed" (see link below).