Natures
Impish raises Defense by 10% and lowers Sp. Atk by 10%. It is applied at the very end of the formula, after IVs and EVs, so the higher the stat, the more the nature gives and takes.
A physical wall that also hits back: Defense goes up and Sp. Atk, which such a Pokémon rarely uses, goes down.
Flavor
Likes Sour, dislikes Dry. This matters for Poffins, Pokéblocks and Berries: a Pokémon eats a disliked flavor reluctantly, and healing works worse.