Professor Cerise was discussing Pokémon Eggs with Ash Ketchum and Goh's Flygon and they decided to go looking for some in Vermilion City. After a Pokémon World Championships Normal Class battle in a park in Vermilion City against Hayden, Ash Ketchum sensed a strong aura wave calling him. When they went to the Pokémon Center to tend to Pikachu, the aura wave strengthen and he followed it to another room where a Pokémon Egg was in an incubator.

Nurse Joy told him the egg was entrusted to her by a Pokémon Day Care. The egg was discovered in the Sinnoh region and it was cared for by many people before it ended up with her. She then suggested Ash Ketchum care for it as he seemed to have a connection to it. One night while he was sleeping, the egg hatched into a Riolu. It jumped through the bedroom window and began fighting wild Pokémon in the area. It kept getting beat up and then it faced an Onix but Ash Ketchum arrived to help him battle the Onix which then caused the Onix to flee the battle.

After being fixed up by Nurse Joy, they all recognized it was a pretty stubborn Pokémon but it indicated that it wanted to be caught by Ash Ketchum. He threw a Poké Ball and was quickly caught.

Ash's Lucario is the evolved form of Ash's Riolu. It evolved during the battle against Chairman Rose's Copperajah and Chairman Rose's Ferrothorn at the Hammerlocke Stadium.