In The Road to Humilau!, Cameron and Riolu joined Ash and his friends to travel to the Humilau Gym so Cameron could get his final Gym Badge. After getting lost and separated from Cilan and Iris, Riolu, Cameron, Ash, Pikachu, and Meloetta fled into a cave to shield themselves from the pouring rain and stayed there for the night. Later, they met Marlon, the Humilau Gym Leader, who took them to the Humilau Gym so that Cameron could have his Gym battle. Although Cameron wanted to use Riolu in the fight, he accidentally left it on the side, forcing him to use his other Pokémon to defeat Marlon. Afterwards, Riolu and Cameron went their separate ways from Ash and his friends.
Later, Riolu reappeared with its Trainer at Vertress City, where they camped out near the building where the Vertress Conference was being held. After Cameron revealed he forgot to enter in the tournament, Ash dragged the two to the building to register. In Mission: Defeat Your Rival!, Riolu was used against Bianca's Emboar in the Vertress Conference. Despite the much larger size of its opponent, Riolu was able to gain an advantage by using Circle Throw to use Emboar's weight against him. Emboar quickly regained the advantage in the fight, but Riolu countered by copying his moves with Copycat. Eventually, Riolu defeated Emboar with Vacuum Wave, making Cameron the victor of the battle.
In Cameron's Secret Weapon!, Riolu was used against Ash's Unfezant in the Vertress Conference. Despite being at a disadvantage, Riolu easily defeated the Proud Pokémon. Next, it faced Ash's Snivy and managed to quickly overpower her. However, Snivy made a comeback and countered all of Riolu's attacks, eventually sending it back with a powerful Leaf Storm. This caused Riolu to evolve into a Lucario, making it much stronger. The battle continued, and Lucario was able to use its new strength to easily overpower Snivy before knocking her out with its newly learned Aura Sphere. Lucario then battled with Ash's Pikachu. The damage Pikachu took from battling Samurott and Swanna quickly put him at a disadvantage, causing Lucario to defeat Pikachu with Aura Sphere and win the battle for Cameron.
In the semifinals, Cameron found himself up against Virgil and his team of Eeveelutions. The final battle came down to between Lucario and Virgil's Flareon; however, Lucario had already been weakened by Virgil's Espeon and was quickly finished off by Flareon's super effective Fire Blast, eliminating Cameron from the competition.
Personality and characteristics
Since its debut, Lucario tended to be annoyed by its Trainer's forgetfulness. Despite this, it is friendly, and gets along well with other Pokémon as well as with its Trainer, which can be attributed to the fact that it evolved from Riolu, who evolves from friendship. In battle, Lucario is a very powerful fighter, and was able to take down opponents much larger than it even as a Riolu, such as Bianca's Emboar and Kenton's Gigalith. The same also applies for the ones who are fully evolved.
Lucario, as a Riolu, tends to copy its Trainer. This was best demonstrated during its debut, and in Lost at the League!, when Russet's family went running by which destroyed their tent even though they worked hard on building it. Riolu continued to copy Cameron even after evolving, as shown when both of them were having ice cream after their battle with Ash.