Pokémon Journeys has provided Ash Ketchum with his greatest challenge yet in the Masters Eight Tournament that will determine the winner of the World Coronation Series. The eight combatants are Ash, the Champions from every known region and Alain, the trainer who defeated Ash in the Kalos League Finals. However, the rich history of characters in the long-running series means that even with such a stacked lineup for the Masters Eight, things could have been much tougher if two particular trainers had entered the WCS as well.
Legendary Pokémon are exactly what their classification implies: so powerful and rare that capturing even one is nigh-impossible -- and if the right trainer wields such a Pokémon, they are virtually unstoppable. Other than Ash’s Melmetal – a Mythical Legendary he keeps at Kukui’s Lab and hasn’t used since he left Alola – there are no known Legendary Pokémon possessed by any of the Masters Eight competitors. Ash has had to face down Legendary Pokémon in the past and struggled mightily against them, making it extremely fortunate that neither Brandon nor Tobias are among the trainers in the Masters Eight.
Brandon the Pyramid King
The Frontier Brain in charge of the Battle Pyramid, as well as the most powerful (duh) member of Kanto’s Battle Frontier, Brandon possesses not one, not two but three Legendary Pokémon -- the Legendary Titans Registeel, Regirock and Regice. Brandon’s Legendary Trio has a long history of crushing dreams and taking names in the anime. Brandon used them to make a fool out of Paul -- Ash’s greatest rival -- and had previously defeated Paul’s brother Reggie so badly that he gave up on being a Pokémon trainer altogether. Ash, on the other hand, needed three tries to defeat Brandon, with Pikachu barely managing to bring down Regice after a prolonged and dramatic battle.
It's highly unlikely that Brandon participated in the World Coronation Series for two reasons. First, if he had, he’d have made the Masters Eight. Second, in his last appearance in the anime, he decided to settle down in the Sinnoh Region so his Regis could serve as the new Pillars to guard Regigigas at Snowpoint Temple. All signs indicate that the Masters Eight will consist of one-on-one battles, and defeating even one of Brandon’s Regis is a tall order. Ash’s Mega Lucario would hold a significant Type advantage over any of them, but they aren’t called Legendary Pokémon for nothing. Each time Brandon previously defeated Ash, he did so at a severe Type disadvantage, as his Regirock defeated Ash’s Sceptile while his Registeel beat Ash’s Torkoal.
Tobias the Action Replay Man
Much like his signature Pokémon, Darkrai, Tobias is a walking nightmare. Ash’s date with destiny in the Sinnoh League was derailed by Tobias’s Dark Void-spamming Darkrai and his devastating Latios. Considering his only known Pokémon are Darkrai and Latios, there’s no way to know what other Pokémon this man has. It’s not known when, where or how Tobias caught these two Legendary Pokémon, but he was said to have defeated every Gym in Sinnoh and every opponent in the Sinnoh League with just his Darkrai -- only Ash forced him to use a second Pokémon. While Ash did manage to defeat each of them, it took four of his Pokémon to bring down Darkrai and three just to tie with Latios -- the two Legendary Pokémon actually one-shotted most of Ash’s team.
If the Masters Eight does end up being one-on-one battles, Tobias would be one of the most difficult opponents imaginable considering how outrageously overpowered his two known Pokémon are. However, Tobias may not be quite so undefeatable as the Sinnoh League made him seem. After winning the Sinnoh League, Tobias would have gone on to challenge the Elite Four and Cynthia for the title of Champion, the result of which is fairly obvious considering that Cynthia is still the Champion. Tobias hasn’t reappeared since the Sinnoh League, and it’s not known if he participated in the World Coronation Series. Thankfully, the anime chose to bring back the other trainer who crushed Ash’s dreams in a controversial victory -- but without siccing multiple Legendary Pokémon on him.