At the start of the game, you are asked a number of questions to determine the Pokémon that you become. This is done based on Personality and gender. Below are the Pokémon that your main character becomes:

Picture No. Name In-Game Pic
#001 Bulbasaur
#004 Charmander
#007 Squirtle
#025 Pikachu
#052 Meowth
#054 Psyduck
#066 Machop
#104 Cubone
#133 Eevee
#152 Chikorita
#155 Cyndaquil
#158 Totodile
#252 Treecko
#255 Torchic
#258 Mudkip
#300 Skitty

However after that, you also get a choice of a Partner Pokémon. This Pokémon is decided based upon your main Pokémon. You get to choose out of the following Pokémon, HOWEVER...you cannot pick a Partner that is of the same type as your main Pokémon. For example if you get Psyduck, you are unable to choose Squirtle, Totodile or Mudkip.