This Riolu is a fighting-type Pokémon owned by Riley.
Riolu was still in its egg when it was stolen by two men who were hired by Team Galactic. Riley and Diamond went to retrieve the egg but Riley and his Lucario were blinded by honey and also couldn't use their aura to locate the two men and the egg. Diamond eventually defeated the two men and retrieved the egg. The egg soon hatched into a Riolu. It was then seen that Lucario and Riolu could get along with each other very quickly after.
None of Riolu's moves are known.