As with all Battle facilities, there are a number of trainers who you will battle throughout the Battle Tree. These trainers offer varying challenge and if you face them multiple times, you will encounter. These trainers have a massive pool of Pokémon from which their Pokémon are drawn and the number they use varies depending upon which type of battling you are doing. Single Battle draws three, Double draws four, Multi draws two. This means the combinations of Pokémon you encounter in the Battle Tree are endless.
In addition to this, you can now Scout each trainer and have them become a Multi Battle partner. Doing this will have them use their first two Pokémon.
All abilities used by Pokémon are random from their ability pool.