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. Not wanting Lucario to be present for his sacrifice, Aaron threw his staff, sealing Lucario within. The armies raged past the staff, and Aaron's Pidgeot later returned it to Rin. After sealing Lucario, Aaron sacrificed himself to the Tree of Beginning to stop the war, and he succeeded.
Lucario's final words to Ash
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. Still blinded from the attack years ago, Lucario initially mistook Ash for Aaron, but he regained his sight, but was stubborn and distrustful of humans after being sealed in the staff. Unaware of Aaron's sacrifice, Lucario held a grudge toward Aaron, thinking that 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.
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 and give his own life, 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 was later seen with Aaron once again. He gave Aaron a bar of chocolate, which he enjoyed, leaving their true statuses unclear, 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.
Lucario was very respectful towards Sir Aaron, seeing himself as a student to his master in learning how to manipulate Aura. However, when Aaron trapped him in his staff just before leaving to sacrifice himself to the Tree of Beginning, Lucario was utterly shocked at his master's "betrayal" and lost all respect for him, as he did not know about the sacrifice.
Due to this experience, Lucario developed a high degree of animosity against all humans, saying that they couldn't be trusted. It wasn't until Lucario realized just how much Ash cared for his Pikachu that he began to trust humans again, developing a strong friendship with Ash in the process. As time went on, Lucario found out the truth behind Sir Aaron and his sacrifice, allowing him to forgive Sir Aaron for misunderstanding. Lucario's death allowed him to be forever at peace, and be with Sir Aaron in the after-life.
Lucario Cards listed with a blue background are only legal to use in the current Expanded format. Cards listed with a silver background are legal to use in both the current Standard and Expanded formats.