To use the inventory, your companion Eloimaya must be following you and not too far away.
To display the inventory, stand in front of Eloimaya and press [I].
To toggle between items and armor, use these buttons. 
To drink a potion, double click on the potion or drag it onto the health bar.
To give a potion to a companion, drag it onto the companion’s health bar.
Dragging an object on the portrait is used to make the object swallow. ( shift to move them by 5 )
To add a potion/arrow in a shortcut, simply drag it ( shift to move them by 5 )
Dragging an object on top of another one allows you to make a new object
To discard an item, drag and drop it outside the inventory.