Natures
Gentle raises Sp. Def by 10% and lowers Defense 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.
Sp. Def at the cost of Defense. Rare, because most walls would rather have both sides solid than one very strong.
Flavor
Likes Bitter, dislikes Sour. This matters for Poffins, Pokéblocks and Berries: a Pokémon eats a disliked flavor reluctantly, and healing works worse.