The two players in the book of Joel are Joel himself and the people of Judah. Little is known about Joel, since he is not mentioned outside this book and in Acts 2:16. There are 12 other men named Joel in the Bible, but none of them can be properly identified as the prophet Joel, writer of this book.
Since Joel is addressing the people in the southern kingdom of Judah, it is not surprising to see a particular focus on Jerusalem. This great city, the capital of Judah, is directly mentioned six times in the book's three chapters.