Key items are the essential items that you find throughout your journey. They include items like the Vs. Seeker, Poffin Case, Explorer’s Kit, and Bike. Each of these items can be registered so that you can use them without needing to access your inventory. In previous games, you were only able to register a single item. That number has now been increased to four. Here’s how you register and use these items in Pokémon Brilliant Diamond and Shining Pearl.
To begin the process of registering an item, you need to access your inventory by pressing X and scroll over to the key items tab. Once there, select the item that you wish to register by pressing the A button. After selecting the item, you will see an option to register it. Select register and another menu will pop up so you can choose which direction it should be registered to.
Now that the item has been registered, you can access your registered item menu by pressing the + button. The same menu that appears when you register an item will appear. Press the direction of the item you wish to use with either the D-Pad or the left stick. For items like the Bike, you can go back to this menu and select the item again to dismiss it.