
Perhaps he is coming home from a hard business day in the 1960s. He could be a criminal (you know, one of those gangster types) walking to a court case in the late 1800s. The image could even be that of a young girl dressing up like Mary Kate and Ashley Olsen in the mid 1990s. As clear to any witness to this image, the figure here is invisible. Something described as invisible is difficult, if not impossible, to define. But every thought, every identity, you give to this image makes it easier to describe.
Calvino write an entire novel (whether or not Invisible Cities is one is debatable) about several cities described as invisible. He uses what he knows about people, landscapes, culture, etc. to fill in the picture. For sake of the story line, how can Marco Polo convince Kublai Khan of the reality of "fake" cities? I do not believe that Polo's point here is to tell the emperor about cities in his empire. Marco Polo wants to convince the emperor that to rule a people, one must know the people. By 'filling in the picture' Kublai Khan will better rule his empire. Polo is trying to make this point subtly in order to avoid offending Kublai Khan. You know, make the emperor feel like he came up with the idea himself to submerge himself in the life of a commoner. The task is explained easily when Polo says,
"At times all I need is a brief glimpse, an opening in the midst of an incongruous landscape, a glint of lights in the fog, the dialogue of two passersby meeting in the crowd, and I think that, setting out from there, I will put together, piece by piece, the perfect city, made of fragments mixed with the rest, of instances separated by intervals, of signals one sends out, not knowing who receives them."
In order for the great Kublai Khan to rule over his empire successfully, he must visit the cities himself and gain a greater knowledge of his people.