Hearts

Role
Hearts are an essential item in each box. Four of them are required to open up the door to the next level. Hearts can be found in bottles.

The hearts could be symbolic for friendship being the key since they open the doors in every level. However, this is never stated in-game.

Physical Appearance
Hearts are exactly the shape you'd expect them to be; heart shaped. They have a golden texture judging by the white shine coming off of them.

Trivia

 * The Hearts are used to defeat Majyo in the final box.