Riolu or No.115 is a Fighting-type Pokemon that can evolve into a Lucario. To get Riolu, you are going to need to help Riley with his quest first. This guide will show you how you can get Riolu or No. 115 in Pokemon Brilliant Diamond and Shining Pearl.
Help Riley Out
To get Riolu, you need to go to Iron Island. To do so, go to the Canalave City and talk to the Sailer, he will bring you to the Iron Island. After arriving, go to the north until you reach the entrance to the cave.
Go down the staircase to the right and follow the path until you reach an elevator that takes you to the lower level. Go down the left staircase this time and you will meet Riley at the entrance of the cave.
Explore the cave with Riley and keep going south until you meet the two grunts next to the elevator. Defeat them and Riley will give you an egg. You can now put the egg in your party and wait for it to hatch into Riolu after a few steps.