Congrats you receive a diploma from the game designer at the celadon mansion, you receive a gold trainer card and show your dex to oak and he will... roar at you.
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Complete the Unova pokedex (after that coplete the national pokedex)
Just a certificate saying that you completed the pokedex.
if you own a Pokemon lets say magikarp the red areas on the map on the pokedex are where you can find it.
No it isn't cheating because that's how you complete your Pokedex in Pearl/Diamond/Platnium.
Once you have completed the pokedex I think you should try and evolve them all and then get them all to level 100 although its impossible to complete the pokedex without an action replay