Riolu evolves into Lucario when it levels up with high friendship (happiness) during the daytime. This means you need to raise Riolu's friendship by keeping it in your party, winning battles without fainting, and giving it certain items or massages that increase friendship. Once the friendship level is sufficiently high, Riolu will evolve upon gaining a level in the daytime. This evolution method has been consistent across Pokémon games since Riolu's introduction in Generation IV.