Pokémon Ranger and the Kidnapped Riolu! (1) - S11.E19

On their travels, Ash and his friends meet up with another Pokémon Ranger—Kellyn—and a Riolu. It turns out that Hunter J is after Riolu, due to its ability to use Aura Sphere before evolving. Ash and Riolu start to bond with their shared Aura, but J is insistent on getting it. This episode relates to the secret mission in Pokémon Ranger: Shadows of Almia, from which the player can transfer a Riolu that knows Aura Sphere to their copy of Diamond, Pearl, or Platinum.