When Lucario and Maylene first appeared, Lucario was angry with Maylene for losing her fighting spirit after being easily defeated by Paul and kept on attacking her. However, Dawn told Maylene that Lucario was only angry because it was worried about her, proven that while stern, it cares deeply for her and will not tolerate people talking negatively about Maylene, especially shown when it sends Team Rocket flying for saying how pathetic she was. Lucario and Maylene were very close, despite the fact that they could get into fights. Lucario was almost always seen outside its Poké Ball.
When Ash challenged Maylene, Lucario was the last Pokémon Maylene used. Lucario defeated Staravia and Chimchar and then battled Buizel. During the battle, Buizel learned Water Pulse and when Lucario's Aura Sphere collided with Buizel's new attack while they were up near the ceiling, the resulting explosion blew a huge hole in the roof. It then started raining, causing Buizel to activate Swift Swim. Although Buizel's speed was increased by its ability, Lucario still managed to keep up with Buizel and in the end, the battle finished in a draw.