As in real life, cultivating friendship in Pokémon Brilliant Diamond and Shining Pearl takes time and effort. There are many ways to quickly increase friendship with a Pokémon, such as keeping them in your party and not letting them faint. Once you reach Hearthome City, however, two major friendship-builders open up to you. One is Amity Square, which unlocks the ability to take a Pokémon for a stroll outside the park. The other is the ability to make Poffins and feed them to your Pokémon.
The usefulness of Poffins is eventually imparted to you by Dr. Footsteps, whose house is on Route 213, just southeast of Pastoria City. Dr. Footsteps will tell you what the first Pokémon in your party thinks of you, and he’ll hint as to how your friendship can be improved. If he says your Pokémon is hungry, that means your bond can deepen if you feed your Pokémon some Poffins.
Once you get the Poffin Case, you can make as many Poffins as your thumbs can handle, and you can feed your Pokémon Poffins until it says it’s full. Check your Pokémon’s summary to see if they like a particular kind of Poffin — spicy, sour, etc. If you feed them the specific kind they like, they’ll be happier, which means your friendship will improve more quickly.
Some Pokémon will “eat anything,” which is okay, too. Feeding your Pokémon any Poffin (except Foul, possibly) will improve your bond, but catering the selection is like icing on the cake — or, rather, sprinkles on the Poffin.