There are three main characters really. The little girl is the one who starts the whole situation by crying wolf when there isn't one. Then there are the villagers who respond to her cries at first but then stop believing her. And the wolf, which is the real danger that the girl fails to get help for when it actually arrives.
The main characters are the little girl and the villagers. And of course, there is the wolf.
The key characters are the little girl, who is the protagonist of the story and whose actions drive the plot. The villagers play an important role as they are the ones who are deceived by the girl's lies. And the wolf is the antagonist, the threat that the girl should have been warning the villagers about truthfully.