Sometime after Sorrel lost his Luxray, Lucario appeared in his life and helped him overcome his traumatic loss of Luxray.
Sorrel used his Lucario to attack Entei with Aura Sphere after it attacked Ash and Verity but Entei decided to attack it back. When sheltering for the rain, Sorrel decided to recall Lucario to keep it safe but Lucario came back out, not wanting to leave Sorrel alone. Lucario then accompanied the trio, who were on their way to meet Ho-Oh since Ash had the Rainbow Wing.
When they arrived at the mountain, Cross stole Ash's rainbow feather after he lost his battle with Ash. The feather became black after Cross put it on the Rainbow Rock, causing Marshadow to appear, and it manipulated several Pokémon, which started to attack. Lucario blocked an attack from Pidgeot, who started to attack Bonji. Lucario then protected Ash from the rampaging Pokémon with Bone Rush, so that Ash was able to reach Marshadow to get the feather back. Lucario was happy when Ash came back and Ho-Oh arrived after Ash put the feather back, healing the damage Lucario has sustained from the battle. After the event, Sorrel and Lucario seperated from Ash and Verity since Sorrel wanted to know more about Articuno, Moltres and Zapdos.