Lucario is a Fighting and Steel type Pokémon known as the Aura Pokémon who serves as a central character in the film. Hundreds of years before the main events, Lucario was the loyal apprentice and partner of Sir Aaron, a nobleman who could wield a mysterious power called Aura. When a great war threatened the kingdom of Rota, Aaron and Lucario were sent to stop the conflict. On the way to the battlefield, Aaron told Lucario he was leaving the kingdom and would not return, then unexpectedly sealed Lucario inside the jewel of his staff. Lucario was trapped in that state for centuries, unaware of what happened next. After the sealing, the Tree of Beginning shone with light and the war ceased, and Aaron was hailed as a hero. Through the ages, the story of the Wave-Guiding Hero was passed down as a legend.

When Ash Ketchum wins a tournament at Cameron Palace and is granted Aaron’s staff, his own aura energy releases Lucario from his long imprisonment. Upon emerging, Lucario is angry and bitter towards humans, especially Aaron, believing his master betrayed and abandoned him. He is distrustful, proud, and openly hostile, refusing to cooperate with Ash and his friends. Lucario’s motivation throughout the film is to understand why Aaron sealed him away and to seek the truth behind his master’s actions. He reluctantly agrees to guide Ash to the Tree of Beginning after Mew takes Pikachu and Meowth there, but his emotional journey is centered on resolving his feelings of betrayal.

Lucario’s role in the story is both a guide and a reluctant ally. He possesses the ability to sense and manipulate aura, a form of life energy, which allows him to read the emotions and intentions of others and to perceive events from a distance. He can also understand human speech and communicate telepathically with humans, though he chooses to speak aloud. In the Tree of Beginning, Lucario’s aura abilities are essential for navigating the dangerous environment and interpreting the time flowers, which are special plants that can replay visions of the past.

The key relationship in Lucario’s life is with Sir Aaron. Initially, Lucario respects and follows Aaron without question, but his misconception of Aaron’s actions leaves him feeling lied to and discarded. The most important development occurs when Lucario and Ash find a time flower that reveals Aaron’s true sacrifice: Aaron gave his own aura energy to the Tree of Beginning to end the war, and he sealed Lucario away specifically to prevent him from dying alongside him. After learning this, Lucario forgives Aaron and reconciles with his memory. A second key relationship forms with Ash Ketchum. Ash’s own aura is similar to Aaron’s, and through their shared journey and a physical confrontation, Ash earns Lucario’s trust. They work together to combine their aura to stabilize the Tree of Beginning and save Mew. At the climax, Lucario pushes Ash out of the way and completes the aura restoration himself, an act that drains his own life. He then sees one last time flower vision of Aaron’s final moments, where Aaron thanks Lucario for being his friend. With this closure, Lucario peacefully passes away.

Notable abilities include aura sensing, aura manipulation, and fighting capability. Lucario can read the aura of living things to determine their true intentions, and he can project aura as a weapon or a shield. He is also a skilled close-combat fighter and can withstand significant damage. His sensitivity to aura also allows him to track Mew and locate Pikachu within the vast Tree of Beginning.