Who is Madame Sina Marnis from Eleven Short Stories = Undici Novelle and what is their importance?

Madame Sina Marnis is also called Teresina. She is a popular singer who started off a poor Sicilian girl. She was encouraged by Micuccio to pursue a career in singing and left him with the promise to marry him after five years. She is self-centered and selfish, however, and abandons all thought of marrying Micuccio after she finds success and popularity.