In the Pokmon Black and White 2 games, Riolu evolves into Lucario when it levels up during the day with high friendship.
Friendship is a mechanic in the Pokmon games that measures the bond between a Pokmon and its trainer. It can be increased by battling, using items, and talking to the Pokmon. When a Pokmon has high friendship, it will obey its trainer even if it is at a low level, and it will be more likely to use powerful moves in battle.