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.
Find which nature corresponds to your preferred stat boosts/drops.
|- Attack||- Defense||- Sp. Atk||- Sp. Def||- Speed|
|+ Sp. Atk||Modest||Mild||Bashful||Rash||Quiet|
|+ Sp. Def||Calm||Gentle||Careful||Quirky||Sassy|
Check which stats are raised/lowered by your Pokémon's nature.
|Bashful||Sp. Atk||Sp. Atk|
|Careful||Sp. Def||Sp. Atk|
|Quirky||Sp. Def||Sp. Def|
|Rash||Sp. Atk||Sp. Def|