Lucario was caught as a Riolu in B2W2-013. Lexi was in Nimbasa City when a horde of Riolu went on the loose. After successfully getting them all back together and releasing them into the wild, one Riolu stayed behind wanting to join Lexi. She accepted, and it joined Lexi on her team. It stayed with Lexi until she left Sinnoh for Kalos. In this time, he was apart of Lexi's teams while fighting against Elesa, Skyla, Drayden, along with both the Unova League's Marshal and her fight against Team Plasma, becoming a vital part of her team during her second journey in Unova.
Riolu appeared once again when Lexi returned to Sinnoh along with the other members of Lexi's Team. It evolved in SI011 after a training session, becoming Lucario.
Lucario returned to the team in Kalos after Lexi in The Sycamore Stone!. Lexi brought him back after receiving a Mega Stone called a Lucarionite that Mega Evolves Lucario into Mega Lucario. He became the 'powerhouse' of Lexi's team once more; however, due to his ability to Mega Evolve, Lucario became unstable while in his powerful state, loosing during Lexi's battle with Ramos and again during Lexi's battle with Valerie.
Despite loosing in the latter battle, he became in control of his strength and ultimately accepting of itself as Mega Lucario.
It battled against Wulfric in Lexi's final Gym Battle and won, becoming its first Gym battle win in Kalos. Lucario was also a main fixture in Lexi's team during her fight against Team Flare.
Lucario has the ability to sense the Auras of all things and understand human speech perfectly. He is able to take in the feelings of creatures over half a mile away and no foe can remain invisible to him.