Catch every Pokemon in the kanto dex, im not sure if this includes mew, but i don't think it does. This is extremely hard to do.
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In Pokemon Platinum to get the National Dex you will need to have seen all 210 Pokemon in the Sinnoh Dex then talk to Professor Rowan and then after a while Professor Oak will give you the National Dex!
After you beat the elite four, get the national dex from professor oak, and go to the new islands. after you do that, the man guarding the cave should be gone.
After seeing all the Pokemon in the Sinnoh Dex go talk to Professor Rowan and Professor Oak will come and give you the pokeradar as well as the National Dex.
all you do is get the national dex, talk to professor oak and he will let you pick one of the three johto starter pokemon. get every Pokemon in the national pokedex and show prof. oak. then you can pick between it, cindaquill, and todadile.
Talk to Professor Oak after meeting three requirements: 1. Register at least sixty (60) Pokémon in the Kanto Dex. 2. Rescue Lostelle from the Berry Forest on Three Island. 3. Enter the Hall of Fame.