When Lucario and Maylene first appeared, Lucario was angry with its trainer for loosing 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. Lucario and Maylene were very close, despite the fact that they could get into fights. Lucario was almost always seen outside its pokeball.
Dawn helped Maylene regain her fighting spirit by challenging her to a battle. Lucario was the second Pokemon Maylene used and defeated Dawn's Ambipom and Piplup.
When Ash challenged Maylene, Lucario was the last Pokemon the Gym Leader 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 Buizels 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 the Water type and in the end the battle finished in a draw.
Lucario's final appearance was when it was used to battle Team Galactic, when the arrived in Veilstone to steal meteorites.