Inventory is where tamers could find and use the items they obtained throughout the game. Inventory could be found in the tamer's cave. The items are sorted in a different category depending on its usage. When the inventory is full the tamer would be restrained from obtaining new type of items. To prevent the inventory from getting full, tamers are encouraged to expand their inventory using gold or selling/using to get rid of unwanted items.
Only one category could be expanded at a time. The price of expansion increases greatly the more it is expanded. Gold is currently the only currency able to expand, while diamonds were allowed for a short period of time before being scrapped off.