No. In PokéPark Wii: Pikachu's Adventure, you are permanently locked to Pikachu as the main protagonist.
Playable Friends: You unlock 19 other Pokémon (Bulbasaur, Charmander, Squirtle, etc.) by befriending them.
Temporary Switch: Press 1 Button (Wii Remote) → "Switch Pokémon" to control a friend for specific attractions/skills (e.g., Bulbasaur's Vine Swing, Garchomp's Fly).
Story Progression: Always reverts to Pikachu for cutscenes, zone transitions, and main objectives.
Think of friends as ability tools, not alternative mains.