Riolu is a Fighting-type Pokmon introduced in Generation IV. It evolves into Lucario when it levels up with high friendship during the day.
In Pokmon Black 2, there are a few ways to evolve Riolu into Lucario. One way is to level it up to level 50 or higher during the day with high friendship. Another way is to use a Dawn Stone on Riolu. Dawn Stones can be found in the wild or purchased from certain shops.