Here's why:
* Trainer is a single character: In Brawl, the Trainer character is a single entity, not a customizable character that allows you to switch between different Pokemon.
* Pokemon are his companions: The Pokemon he uses (Squirtle, Ivysaur, and Charizard) are his companions, and he uses them during the fight.
* No character customization: Brawl doesn't have a system for switching characters within a fight or customizing a character to use different Pokemon.
To change Pokemon, you'll need to:
* Select a different character: If you want to play as a different Pokemon, you'll need to choose a different character entirely. For example, you could play as Pikachu, Lucario, or Pichu.
* Play a different game: In later Smash games, such as Super Smash Bros Ultimate, you can play as individual Pokemon characters instead of the Trainer.
Let me know if you have any other questions about Super Smash Bros Brawl!