Pokémon natures are another key feature of the games, introduced in Generation 3 (Ruby/Sapphire), which can increase certain stats. Each Pokémon has one nature, randomly selected from a total of 25.
Rather than just being a superficial personality, Natures actually affect the growth of a Pokémon. Each nature increases one of its stats by 10% and decreases one by 10% (by the time it reaches level 100). Five natures increase and decrease the same stat and therefore have no effect.
In most cases it is preferable to have a nature that decreases either Attack or Special Attack for Pokémon whose strengths are the opposite type of attack. Espeon, for example, favours Special moves, so it's best to use a nature that decreases its Attack since it won't be used.
A Pokémon's nature also determines the berries it likes and dislikes. Each type of berry is linked to one stat:
The berry a Pokémon likes is the corresponding flavour of its raised stat, while the berry it dislikes is the flavour of its lowered stat.
For example, a Pokémon of Sassy nature will like Bitter berries (Special Defense is raised) and dislike Sweet berries (Speed is lowered).
