Do you want to complete the National Pokédex and unlock the Buddy Park so you can transfer your Pokémon from Ruby, Sapphire, FireRed, LeafGreen to Pokémon Platinum? Well, you'll have to "see" each Sinnoh Pokédex and then go to Prof. Rowan. Most Pokémon can be found fighting with trainers in the Sinnoh region, but to find some you'll need to explore more. Read how to see them all.
Steps
Step 1. Play the main storyline until you beat the Elite Four
On your way, defeat all the trainers you find. You will be able to see almost any Pokémon this way.
Step 2. Go to the Old Castle at night to see Rotom, if you haven't already
The Old Castle is located in the Eternal Forest of the north and it takes Cut (HM01) to get there. Interact with the TV that is there to start a fight with Rotom.
Step 3. Go to the Solaceon Ruins to see Unown, if you haven't already
It is located east of the City of Solaceon.
Step 4. Go to the Pokémon Mansion to see Manaphy, if you haven't already
Read the book in Mr. Backlot's room and Manaphy will be added to your list of seen Pokémon.
Step 5. Visit all the lakes caves again to see the Lake Trio, if you haven't already
You will have to fight with Azelf and Uxie, while you will only have to interact with Mesprit.
Step 6. Go to Prof
Rowan in her lab once you have "seen" each Pokemon in the Sinnoh Pokédex to complete the National Pokédex.
