As with all Battle Facilities found throughout the previous number of Pokemon Games, there are a number of random trainers who you will face off with throughout all modes of The Battle Tree. Each of these trainers offer different challenges, and if you face them multiple times, odds are you'll encounter different Pokemon of theirs. There is a large pool of possible Pokemon for these trainer - their abilities will be at random - the number of Pokemon and which certain ones are used dependent upon the type of battle: Single Battles draw three Pokemon, Doubles draws four, and Multi Battles draw two. The combinations of Pokemon you'll encounter at the Battle Tree overall in endless.
But when you go through The Battle Tree, like all other facilities, you will come across specific trainers that you have encountered during the game in one way or another. These are Special Trainers, such as Dexio, Sina, Grimsley, and Colress, and you will encounter them every ten battles. (There will also be Super Battles after every twenty wins.) The goal is to get to the end of each Battle Mode and battle the Battle Legends, either Red or Blue.