Natures

Each individual Pokémon has a Nature that influences its stats. This Nature is randomly determined upon encountering the Pokémon and cannot be changed legitimately once caught. Each Nature raises one stat by 10% and decreases another stat by 10%. No Natures influence HP, and certain Natures don't affect stats at all.

Prior to catching them, a Pokémon's Nature can be changed by having the Synchronize ability on the first Pokémon in your party, setting the Nature to be the same as the Pokémon with the ability.

After catching it, the nature can only be changed by /pokeedit or similar commands, but can also be effectively changed by using a Mint on it, which changes the stats boosted and reduced to reflect another Nature instead.

Within NBT tags, Natures are stored as numerical IDs. These numbers are only useful when editing Pokémon NBT data externally.

Nature	Increased stat	Decreased stat	ID
Adamant	Attack	Special Attack	7
Bashful	None	None	3
Bold	Defense	Attack	9
Brave	Attack	Speed	6
Calm	Special Defense	Attack	21
Careful	Special Defense	Special Attack	24
Docile	None	None	2
Gentle	Special Defense	Defense	22
Hardy	None	None	0
Hasty	Speed	Defense	14
Impish	Defense	Special Attack	11
Jolly	Speed	Special Attack	15
Lax	Defense	Special Defense	12
Lonely	Attack	Defense	5
Mild	Special Attack	Defense	18
Modest	Special Attack	Attack	17
Naive	Speed	Special Defense	16
Naughty	Attack	Special Defense	8
Quiet	Special Attack	Speed	19
Quirky	None	None	4
Rash	Special Attack	Special Defense	20
Relaxed	Defense	Speed	10
Sassy	Special Defense	Speed	23
Serious	None	None	1
Timid	Speed	Attack	13