Lucario first appeared as a Riolu with its Trainer in Goodbye, Junior Cup - Hello Adventure!. They revealed that they had come to enter the Pokémon World Tournament Junior Cup only to find out that it had already ended, much to Kotetsu's displeasure. Later, Kotetsu and Riolu revealed to Ash and his friends that they planned on entering the Unova League, but only had seven Gym Badges.
In The Road to Humilau Kotetsu and Riolu joined Ash and his friends to travel to the Humilau Gym so Kotetsu could get his final Gym Badge. After getting lost and separated from Cilan and Iris, Riolu, Kotetsu, 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 Kotetsu could have his Gym battle. Although Kotetsu 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 Kotetsu went their separate ways from Ash and his friends.
Later, Riolu reappeared with its Trainer at Higaki City where they camped out near the building where the Higaki Conference was being held. After Kotetsu revealed he forgot to enter in the tournament, Ash dragged the two to the building to register. In BW104, Riolu was used against Bianca's Emboar in the Higaki 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 it. Emboar quickly regained the advantage in the fight, but Riolu countered by copying its moves with Copycat. Eventually, Riolu defeated Emboar with Vacuum Wave, making Kotetsu the victor of the battle.
In BW106, Riolu defeated Kenton's Gigalith, which allowed Kotetsu to progress to the Top 8 of the Higaki Conference.
In BW107, Riolu was used against Ash's Unfezant in the Higaki Conference. Despite the type disadvantage, Riolu managed to defeat Unfezant. Next, it faced Ash's Snivy and managed to quickly overpower her. However, Snivy quickly managed to counter Riolu's attacks. This caused Riolu to evolve into a Lucario, making it much stronger.
Since its debut, Lucario was shown 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 as a Riolu, such as Bianca's Emboar.
