The princess is the primary character in 'the princess and the tin box'. She is at the center of the action, having to choose among various offerings. The tin box, while not a human character, is a crucial element in the story. It represents something different from the other gifts. And though we don't know who exactly gave the tin box, the giver, in a way, is also a part of the story as it influences the princess's decision - she chooses the tin box for some reason related to the giver or what it represents.
Well, the princess is clearly a main character. She is the one making the choice among different items, including the tin box. There may not be many other directly named characters. However, those who presented the gifts (even if not named) are also sort of characters in the background, as they are part of the situation that sets up the princess's decision regarding the tin box.