Natures
Timid raises Speed 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.
Speed above all for a special attacker. Speed tiers decide who moves first — ten points can change an entire matchup.
Flavor
Likes Sweet, dislikes Spicy. This matters for Poffins, Pokéblocks and Berries: a Pokémon eats a disliked flavor reluctantly, and healing works worse.