Natures
Calm raises Sp. Def 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.
A special wall that does not need Attack. The natural pick for self-healing Pokémon that absorb special attacks.
Flavor
Likes Bitter, dislikes Spicy. This matters for Poffins, Pokéblocks and Berries: a Pokémon eats a disliked flavor reluctantly, and healing works worse.