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<<Ruby / Sapphire - Pokemon Natures>>
Another one of the new twists in Ruby & Sapphire battles is the addition of Pokemon Natures / Personalities.  When you catch a Pokemon, it's Nature will be predetermined randomly out of 25  Natures, and each one will have a slight effect on your Pokemon's stats (Attack, Defense, Speed, etc.).  A Nature will raise the growth rate of one stat by a small factor between 1 and 1.5, and it will lower the growth rate of another stat by 0.85 to 1.  It's hardly noticeable so it stays within the set Pokemon's Perameters.

Also to be noted, some Pokemon Natures will raise
and lower only one stat out of your Pokemon's 5.  This cancelation can be known as a stalemate Nature, meaning that the Pokemon doesn't gain or lose any points from any of its stats, which can be either a good or bad thing depending on the Pokemon's best abilities.  Anyway, here's a table showing each Nature and the stats it raises or lowers:
Nature's Name
Hardy
Lonely
Brave
Adamant
Naughty
Bold
Docile
Relaxed
Impish
Lax
Timid
Hasty
Serious
Jolly
Naive
Modest
Mild
Quiet
Bashful
Rash
Calm
Gentle
Sassy
Careful
Quirky
Raises
Attack
Attack
Attack
Attack
Attack
Defense
Defense
Defense
Defense
Defense
Speed
Speed
Speed
Speed
Speed
Special Attack
Special Attack
Special Attack
Special Attack
Special Attack
Special Defense
Special Defense
Special Defense
Special Defense
Special Defense
Lowers
Attack
Defense
Speed
Special Attack
Special Defense
Attack
Defense
Speed
Special Attack
Special Defense
Attack
Defense
Speed
Special Attack
Special Defense
Attack
Defense
Speed
Special Attack
Special Defense
Attack
Defense
Speed
Special Attack
Special Defense
So when a Pokemon has a Nature that raises a stat and lowers another, that Pokemon will have significant differences in stat points when fully trained, as opposed to a Pokemon witha stalemate Nature.  Here's an example:
HP ATK DEF SPD Sp.A Sp.D
Blaziken   lv.100   Adamant   364  372  239  259  287  239
Blaziken   lv.100     Quirky      364  339  239  259  319  239
This example shows 2 fully-trained, level 100 Blaziken.  One is Adamant and the other is Quirky, a stalemate Nature.  As you can see, Adamant Blaziken's Attack surpasses that of Quirky Blaziken, but in the Special Attack, Adamant Blaziken is below par compared to Quirky.
Kyogre's Brave Nature highlighted...
You can locate your Pokemon's Nature in the bottom box on the 1st page of the Pokemon's Stats Screens.
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