instruction: Who is the protagonist in Italo Calvino's Invisible Cities?
context: "The book explores imagination and the imaginable through the descriptions of cities by an explorer, Marco Polo. The book is framed as a conversation between the elderly and busy emperor Kublai Khan, who constantly has merchants coming to describe the state of his expanding and vast empire, and Polo. The majority of the book consists of brief prose poems describing 55 fictitious cities that are narrated by Polo, many of which can be read as parables or meditations on culture, language, time, memory, death, or the general nature of human experience.
  
  Short dialogues between Kublai and Polo are interspersed every five to ten cities discussing these topics. These interludes between the two characters are no less poetically constructed than the cities, and form a framing device that plays with the natural complexity of language and stories. In one key exchange in the middle of the book, Kublai prods Polo to tell him of the one city he has never mentioned directly—h"
response: The book explores imagination and the imaginable through the descriptions of cities by an explorer, Marco Polo.