The inventory is one of the menus that can be access when playing the game. It can be accessed by pressing the bag icon in the upper right or by pressing, "I."
The inventory displays everything the player has collected (with the exception of rare kills, gold, and experience). There are three submenus in the inventory: The Backpack, Spellbook, and Class Skills. These menus can each be viewed by clicking on their respective icons on the left. Above these three icons is the key icon, if there is a key the icon will be present. If the player doesn't own a key then the icon will be a grey silhouette of a key.
The Backpack shows any items the player is wielding or has bought or picked up from looting chests, monster drops, quest items, or quest rewards. In this menu, equipable items can be equipped or unequipped, and items can be sorted onto the hotbar. 48 different stacks of items can be held. In the inventory, players can also view the item's description and other info, use consumables, or destroy them.
The Spellbook is exclusively for Wizard classes. When a spell is picked up or purchased, players can "learn" the spell by viewing the Backpack menu, selecting the spell, and then selecting the "Learn" option. A learned spell is permanent; a player only needs to learn the spell once and can use it as often as they like. Players can learn spells they do not have a high enough level for, but still cannot cast them (level requirements are found on the "info" page for the item.) Spells can be placed on the hotbar.
The Class Skills menu is for Warrior and Archer classes. Skills learned from Ryu at the Training Camp can be viewed, used, or equipped here. Like spells, class skills can also be placed on the hotbar from their menu. Players only need to talk to Ryu once for each skill.