The Index is an encyclopedia found in-game. It is located in the bottom menu of the home screen. The Index shows all the monsters you have encountered. The ones you have encountered in cubes is shown with a bright colour, while those you only have met is showed as dimmed.
The index has five different categories. All, Water, Fire, Nature, and Normal. Besides the categories, the index is also able to sort monsters after Rarity. The monsters in the same evolution tree is located near each other.