Ni no Kuni: Wrath of the White Witch - Take/Give Heart

One of the most basic premises of going to the other world is to help people out who seem to be lacking in a certain emotion. When you clear Deep Dark Woods, you are given the Locket, which can be used to store these emotions. Find someone who is shown to have an abundance of an emotion and you'll be able to store that in your Locket. You can then find someone who is lacking it and use Give Heart to give them that emotion. This is used to progress in the main story, as well as during sidequests.