Over 1,000 years ago, before the invention of Poké Balls, Lucario served Sir Aaron, who in turn served Queen Rin, the queen of Cameran Palace. Lucario and Aaron were sensitive to aura, invisible vibrations that all objects, especially living things, release. This enabled them to "see" objects and living creatures and predict the actions of others even with their eyes closed. Lucario could use aura as an offensive attack in the form of Aura Sphere, in which he would amass a sphere of energy and hurl it at an opponent. Aaron taught Lucario how to refine his sensitivity to aura.
When Cameran Palace was about to be caught in the middle of a war, Aaron was on his way to the Tree of Beginning to attempt to stop the battle when he came across Lucario, who had been temporarily blinded by a group of sand-kicking Houndoom. Aaron told him that he was never going back to the castle. Aaron intended to sacrifice himself to stop the war, but Lucario jumped to the conclusion that he was abandoning the kingdom. Aaron then threw his staff, sealing Lucario within. The armies raged past the staff, and Aaron's Pidgeot later returned it to Leen. After sealing Lucario, Aaron sacrificed himself to the Tree of Beginning to stop the war, and he succeeded.
Years later, in the present time, Lucario was awoken from the staff when it was held by Ash Ketchum, who has a similar aura pattern to Aaron. Lucario initially mistook Ash for Aaron, and was stubborn and distrustful of humans after being sealed in the staff. Unaware of Aaron's sacrifice, Lucario held a grudge toward Aaron, thinking Aaron had abandoned the castle and him. He had considered himself Aaron's servant, and he could not understand the friendship between Ash and Pikachu. However, a time flower message left by Aaron shortly before his death told Lucario the truth, that Aaron considered Lucario a friend, not a servant.
Lucario holding Ash's hands before dying
Once again the Tree of Beginning was disturbed, threatening to destroy the balance between Pokémon and nature. Ash and Lucario rushed to its core and began to use their aura to energize the tree's defense system. Using up their aura would have meant the same fate as Aaron—death. However, just as Ash was about to use up his aura, Lucario pushed him away, instead sacrificing only himself to the tree.
After Lucario's death, the portrait of Aaron in Cameran Palace was altered to include Lucario by Aaron's side, cementing their relationship as best friends and partners in aura manipulation. Lucario is later seen with Aaron once again and gives him a bar of chocolate, which he enjoys; this is surely after his death, as Lucario did not know what chocolate was until Max gave it to him in an effort to earn his trust for the sake of the group.
