To evolve Riolu into Lucario in Pokmon White 2, you must level it up with high friendship during the daytime. Friendship can be increased by battling with Riolu, using it in your party, and talking to it at the Join Avenue. During the daytime, Riolu will evolve into Lucario once its friendship is high enough.
Evolving Riolu into Lucario is important because Lucario is a powerful Fighting/Steel-type Pokmon. It has high Attack and Special Attack stats, and it can learn a variety of powerful moves, including Aura Sphere, Close Combat, and Extreme Speed. Lucario is also a popular Pokmon in competitive battling, due to its versatility and strength.
