Natures
Bold raises Defense by 10% and lowers Attack 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.
The classic physical wall: more Defense, less Attack, which a wall does not use anyway. Do not give it to a Pokémon that returns damage with a physical move.
Flavor
Likes Sour, dislikes Spicy. This matters for Poffins, Pokéblocks and Berries: a Pokémon eats a disliked flavor reluctantly, and healing works worse.