Riolu is a black and blue, dog-like Pokémon nearing two and a half feet. Its appearance is similar to its evolved form Lucario, though there are some differences. Its legs and torso are colored black while its head, lower body and tail are blue. Riolu have large, rounded bumps on the backs of its forepaws, placed in a similar fashion to Lucario's spikes. Riolu also have a black "mask" with two black appendages on either side, and red eyes.
Riolu are sensitive to a special type of energy called Aura as Lucario is. Riolu's evolved form, Lucario, can use these waves for attacks, tracking, or to predict future moves. However, Riolu does not have the full potential of these powers, using them as described in ripples rather than waves. Moves such as Aura Sphere or Detect can not be learned by Riolu due to its inability to fully use the power of Aura. There is at least one exception to this rule, however.
Riolu evolves into Lucario when you level it up during the day with high friendship.
Also, it and Pikachu are the only non-Starter Pokémon in the Mystery Dungeon series.
The Riolu in Pokémon Ranger and the Kidnapped Riolu! is said to have an identical Aura with Ash. In Lucario and the Mystery of Mew (hyperlink needed here), Lucario says that Ash's Aura is the same as Sir Aaron's. Therefore, The Riolu has the same Aura as Sir Aaron.
Although both Riolu and Lucario were released in the same generation, Riolu is counted as a Baby Pokémon.
Riolu's name is simply based on Lucario's name, switching "Rio" with "Lu" and leaving out "Ca".