Like in the previous Dungeon games Your Pokémon gets decided based upon based upon a personality quiz you get given at the start of the game. From this, the game determines the Pokémon you are. Once this has been decided, you can choose your partner.  

Pic: Name: Type 1 Type 2 Male Nature Female Nature
Bulbasaur Grass Poison Lonely Docile
Charmander Fire na Docile Brave
Squirtle Water na Quirky Bold
Pikachu Electric na Brave Hasty
Vulpix Fire na N/A Relaxed
Eevee Normal na N/A Jolly
Chikorita Grass na Calm Quiet
Cyndaquil Fire na Timid Calm
Totodile Water na Jolly Sassy
Phanpy Ground na Relaxed N/A
Treecko Grass na Quiet Hardy
Torchic Fire na Hardy Rash
Mudkip Water na Rash Lonely
Skitty Normal na N/A Naďve
Turtwig Grass na Bold Timid
Chimchar Fire na Naďve Impish
Piplup Water na Impish Quirky
Shinx Electric na Hasty N/A
Riolu Fighting na Sassy N/A

After that, like in previous games, 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 Piplup, you are unable to choose Squirtle, Totodile or Mudkip. The partners possible are as follows;

Pic: Name: Type 1 Type 2
Bulbasaur Grass Poison
Charmander Fire na
Squirtle Water na
Pikachu Electric na
Vulpix Fire na
Meowth Normal na
Eevee Normal na
Chikorita Grass na
Cyndaquil Fire na
Totodile Water na
Phanpy Ground na
Treecko Grass na
Torchic Fire na
Mudkip Water na
Skitty Normal na
Turtwig Grass na
Chimchar Fire na
Piplup Water na
Shinx Electric na
Munchlax Normal na
Riolu Fighting na

 
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