There are roughly 100 chests that you will have to open. There are 3 types of chests:
-Life Shards: These chests will give you experience and help you raise your character's overall health and strength. You also get these shards from defeating enemies. They are the most common, and are a bluish green.
-Outfits: You get pieces of outfits in a sort of gold/white looking chest.
-Majin Outits: There are only 2 of these in the game, and they are very big red chests that only the Majin can open.
To get them all, follow the collectible guide below: