Upgrading to the National Dex
Upon starting the game, you are given the Pokedex by Professor Birch. However, this Pokedex only allows you to record the information of the 202 Pokemon native to the Hoenn region. However, it is possible to upgrade the Pokedex to record the information of all 386 known species of Pokemon, including ones that are not native to Hoenn. When you complete your first trade from "Pokemon FireRed" or "LeafGreen," the Pokedex is automatically upgraded to the National Dex, an advanced form of the Pokedex. The National Dex is able to record information from all 386 Pokemon found across all games, allowing you to fully complete the Pokedex.
Adding Pokemon to the Pokedex
Pokemon can be added to the Pokedex in two ways: seeing them and capturing them. Seeing a Pokemon only adds a partial entry to the Pokedex, namely the Pokemon's name, type and the location where it can be captured. Capturing a Pokemon fully completes that Pokemon's entry in the Pokedex, adding information such as the height, weight and a fact about the Pokemon. To complete the Pokedex, it is not enough to see each Pokemon; you must capture one of each Pokemon as well. You can use any of the catching methods available to you to complete this task.
Ways to Catch Pokemon
There are many methods you can use to capture Pokemon and fill your Pokedex. The most common method is by catching the Pokemon yourself, whether it is in the tall grass in each route, in caves or in oceans and rivers. Additionally, you can trade Pokemon from other Game Boy Advance games, such as "FireRed," "Ruby," "Emerald" and "LeafGreen." In addition, some Pokemon can only be captured in the GameCube games "Pokemon XD: Gale of Darkness" and "Pokemon Colosseum," such as Espeon and Umbreon. These Pokemon can then be transferred over to "Sapphire" once you have completed the story portion of the GameCube game.
Reward for a Full Pokedex
Once you have captured all 386 Pokemon, return to Professor Birch in Littleroot Town in his laboratory. If he is not there, he can also be found on Route 101 just north of town. Upon speaking to Professor Birch, he analyzes your Pokedex and comments on the amount of Pokemon you have found. If you have caught one of each of the 386 Pokemon, you are awarded a certificate from Game Freak, the developer, congratulating you on your accomplishment. In addition, a star is added to your Trainer Card as proof that you have completed your Pokedex.