In 'girl in a box based on true story', the girl in the box is the primary character. Her story is the focus. Depending on the nature of the true story, there could be other important characters. For instance, if the girl was kidnapped and held in the box, the kidnapper is a crucial character. Also, if there were witnesses to her being put in the box or to her discovery, they could be part of the main characters as they play a role in either the mystery or the solution of the situation.
Obviously, the girl herself is a central character. If the story is about her rescue, then the rescuers could also be main characters. They might be police officers, firefighters or just ordinary people who happened to find her. And if there was a villain, the one who trapped her in the box, that would be another key character.