Manhattan is an incoming Columbia grad student who we meet arriving at Penn Station (he has to be taking Amtrak or NJ Transit, but it's not clear from where); at the same moment he becomes the avatar, which gives him amnesia about his personal life. Him being an amnesiac gives other characters an excuse to explain things about New York to him/the reader, which is the structural role in the story I alluded to. All the other avatars were born in the city (Brooklyn, the Bronx, Staten Island) or have lived there a while (Queens) and don't lose their knowledge of themselves.