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This is usually the anal region.
trade, breed, train, and catch them all.
number 152 is rotom and you can catch him by going to the old cheatue
you then: battle all the trainers in sinnoh, then you get all your Pokemon to level 100. then you beat the game.
by completing the shinoh pokedex and going to Prof. Birch and then their should be a cut scene where Prof. Oak comes and gives up the national podex.