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Background Information

Diamond will not introduce any new types, however, new type combinations along with new Pokemon (and new pre-evolutions and evolutions) will be introduced. Pokemon Diamond will be compatible with Pokemon Ruby, Sapphire, FireRed, LeafGreen, and Emerald (meaning, you can transfer your Pokemon from these games). Also, the game will support up to 16 players to connect to each other using the Nintendo DS wireless feature.

Manafi to be Trade Only!
It would seem that the cute water Pokémon that is the star of the upcoming Pokémon Movie, Pokémon Ranger & The Prince of the Sea - Manafi, is unobtainable in Pokémon Diamond & Pearl.

According to the information revealed in CoroCoro, you need to have obtained and completed the mission where you have to regain the Manafi Egg from Team GoGo. After you have regained it, Professor Shinbara in Ranger says that the egg cannot hatch in Fiore but can only hatch in Shinou, the new region found in Diamond & Pearl, and that it must be sent there. So using 2 DS's, and a copy of the new games aswell as your copy of Ranger, you can send the egg across to your party in Pokémon Diamond & Pearl and hatch it there.

So there you go, thats how you obtain Manafi.