Credits:

YeahPotato

How To

First insert Defa's Nature Changer Routine (Guide 5.27)

Add this script to an NPC/item, making sure to replace callasm <offset+1> with the offset where you inserted the Nature Changer routine.

section0:
  msgbox.default <auto>
{
Change the Nature of which 
Pokémon?
}
  special ChoosePartyMon
  waitstate
  if.compare.goto var4 >= 6 <finish>
  special IsSelectedMonEgg
  if.yes.goto <isegg>
  special2 temp1 GetPartyMonSpecies
  bufferPokemon buffer1 0x4001

section1:
  random 24
  copyvar temp0 0x800D
  if.compare.call varResult == 1 <lonely>
  if.compare.call varResult == 2 <brave>
  if.compare.call varResult == 3 <adamant>
  if.compare.call varResult == 4 <naughty>
  if.compare.call varResult == 5 <bold>
  if.compare.call varResult == 7 <relaxed>
  if.compare.call varResult == 8 <impish>
  if.compare.call varResult == 9 <lax>
  if.compare.call varResult == 10 <timid>
  if.compare.call varResult == 11 <hasty>
  if.compare.call varResult == 12 <serious>
  if.compare.call varResult == 13 <jolly>
  if.compare.call varResult == 14 <naive>
  if.compare.call varResult == 15 <modest>
  if.compare.call varResult == 16 <mild>
  if.compare.call varResult == 17 <quiet>
  if.compare.call varResult == 19 <rash>
  if.compare.call varResult == 20 <calm>
  if.compare.call varResult == 21 <gentle>
  if.compare.call varResult == 22 <sassy>
  if.compare.call varResult == 23 <careful>
  if.compare.goto varResult == 0 <section1>
  if.compare.goto varResult == 6 <section1>
  if.compare.goto varResult == 18 <section1>
  callasm <offset+1>
  msgbox.default <auto>
{
[buffer1] started acting [buffer2]!
}

finish:
  release
  end

isegg:
  msgbox.default <auto>
{
That's an Egg!
}
  goto <section0>

lonely:
  bufferstring buffer2 <463DC2>
{
Lonely
}
  return

brave:
  bufferstring buffer2 <463DC9>
{
Brave
}
  return

adamant:
  bufferstring buffer2 <463DCF>
{
Adamant
}
  return

naughty:
  bufferstring buffer2 <463DD7>
{
Naughty
}
  return

bold:
  bufferstring buffer2 <463DDF>
{
Bold
}
  return

relaxed:
  bufferstring buffer2 <463DEB>
{
Relaxed
}
  return

impish:
  bufferstring buffer2 <463DF3>
{
Impish
}
  return

lax:
  bufferstring buffer2 <463DFA>
{
Lax
}
  return

timid:
  bufferstring buffer2 <463DFE>
{
Timid
}
  return

hasty:
  bufferstring buffer2 <463E04>
{
Hasty
}
  return

serious:
  bufferstring buffer2 <463E0A>
{
Serious
}
  return

jolly:
  bufferstring buffer2 <463E12>
{
Jolly
}
  return

naive:
  bufferstring buffer2 <463E18>
{
Naive
}
  return

modest:
  bufferstring buffer2 <463E1E>
{
Modest
}
  return

mild:
  bufferstring buffer2 <463E25>
{
Mild
}
  return

quiet:
  bufferstring buffer2 <463E2A>
{
Quiet
}
  return

rash:
  bufferstring buffer2 <463E38>
{
Rash
}
  return

calm:
  bufferstring buffer2 <463E3D>
{
Calm
}
  return

gentle:
  bufferstring buffer2 <463E42>
{
Gentle
}
  return

sassy:
  bufferstring buffer2 <463E49>
{
Sassy
}
  return

careful:
  bufferstring buffer2 <463E4F>
{
Careful
}
  return

Happy Hacking!