Pokemon nature's

Natures (Japanese: せいかく Personality) are the Pokémon analogue of personalities. They were introduced in the Generation III games and remains a game mechanic in Generation IV. Every Pokémon in these games has one of these 25 natures, listed and described in the section below.
A Pokémon's nature usually affects the growth rate of two of its stats, ultimately increasing one of its stats by 10% and decreasing another by 10%. Natures also determine the Pokémon's favorite flavor and its disliked flavor. Each stat is tagged to a flavor (e.g. Attack-Spicy), and if the nature boosts the stat, the tagged flavor will be the Pokémon's favorite (i.e. Lonely boost Attack, hence a Lonely-natured Pokémon's favorite flavor is Spicy). The opposite also holds true (i.e. Bold hinders Attack, so a Bold-natured Pokémon will dislike Spicy food.)
Every nature represents one of the 25 unique possible combination of stat increase and decrease, thus there are five natures that have no effect on the Pokémon's stat growth (Bashful, Docile, Hardy, Quirky & Serious). These five "neutral" natures are technically natures that increase and decrease the same stat.
From Emerald onwards, a Ditto or a female Pokémon (though increasing evidence points to BOTH genders) that holds an Everstone has a 50% chance of passing its nature to its offspring when at the Pokémon daycare, but only if both Pokémon and the trainer all come from a game in the same language. Also from Emerald onwards, if the Pokémon with the ability Synchronize is leading the party, there is a 50% chance of encountering a wild Pokémon with the same nature. Synchronize does not have this effect on stationary legendaries on Emerald, however it works on virtually all Pokémon in every game released since; the only known exceptions in Generation IV are when Pokémon are received from an NPC, such as the Eevee given out by Bill in HeartGold and SoulSilver, when Pokémon are received in a museum after being resurrected from fossils, or when catching Pokémon during a stroll in the Pokéwalker bundled with HeartGold and SoulSilver games.
Natures also dictate the manner in which Pokémon battle by themselves at the Battle Palace, different natures make the Pokémon use different methods of attacks and change tactics when they are low on health. Also, the five non-increasing/decreasing natures have different battle styles at the Battle Palace. A man in a house closest to the Sunyshore City Heritage Site asks to see Pokémon with different natures (Naive, Serious, and Quirky), and will give the player three Pokétch applications.