A Pokémon’s Nature usually affects the growth rate of two of its stats, ultimately increasing one of its non-HP stats (Attack, Defense, Special Attack, Special Defense, or Speed) by 10% and decreasing another by 10%. Natures also determine the Pokémon’s favorite flavor and its disliked flavor: Each stat is associated with a flavor, and each Pokémon’s favorite flavor is the one associated with the stat increased by its Nature, while it dislikes the flavor associated with the decreased stat. For example, since the Lonely Nature increases a Pokémon’s Attack stat (which is associated with the Spicy flavor) and decreases its Defense stat (which is associated with the Sour flavor), a Pokémon with a Lonely Nature has Spicy as its favorite flavor and dislikes Sour flavors.
In Generations III and IV, Nature is determined by the Pokémon’s personality value. In later games, it is stored in an unrelated variable.