The main character seems to be the little king. But considering it's a box theater story, there could be a narrator who is also a character in a way. The narrator could be guiding the story, introducing the little king and his world. And if there are any events or conflicts in the story, there might be other characters like antagonists who oppose the little king's plans or actions in the box theater story.
Based on the title, the most obvious main character is the little king. However, in a theater - related story, there could be actors or performers who interact with the little king. They might play important roles in the story too, perhaps as friends, enemies or guides for the little king within the box theater setting.
Since we only have the title, we can assume the main character is the little king. There may also be other characters like his subjects, advisors or maybe some magical creatures if it's a fantasy - type story.