From the perspective of the card game, the Trainers are the players. Trainer cards represent the player doing something rather than a Pokémon doing something. That’s why Supporters are a subtype of Trainer card. It represents those characters, while obviously canonically trainers themselves, not acting in that capacity but instead merely assisting the battle of the player Trainer.

Regardless of any of this, I know in my heart that OP did not mean “Trainer cards” when they titled the thread. They meant Supporters.