Few Pokemon not native to Kanto can claim to be as recognizable as Lucario. Its face was all over the place back when the original Pokemon Diamond & Pearl were being marketed. It's featured prominently in multiple seasons of the anime and is even in Super Smash Bros. But before you can unleash this prideful Pokemon in battle, you're going to need to locate its unevolved form: Riolu.
In Pokemon Brilliant Diamond & Shining Pearl, Riolu appears in a single fixed location. You can also find them in the Grand Underground in the post-game, but doing so involves a little luck (and prohibits you from using it before the credits roll). Here's the surefire way to score yourself a small blue brawler.
Where To Find Riolu
During Brilliant Diamond & Shining Pearl's extensive middle stretch of the game, one of the eight gyms you'll need to acquire badges from is in Canalave City. But Canalave City isn't on Sinnoh's mainland, and thus, the Surf HM is required. You can acquire Surf by defeating a lone Team Galactic member standing guard outside a cave in Celestic Town.
Don't worry about missing this part - it's a mandatory part of the story.
Once you've surfed to Canalave City, look for a sailor to the city's southwest. He'll offer to let you hitch a ride to Iron Island. The place is infested with Team Galactic grunts, but you'll have help in the form of fellow trainer named Riley. The nice thing about Riley is that, like other temporary escort NPCs in Brilliant Diamond & Shining Pearl, he'll heal your party after every battle. Let's all take a moment to appreciate the good lad.
Riley's personal mission will compel you to zigzag around Iron Island until you reach his destination, at which point he shows his gratitude by handing over a Riolu Egg.
How To Hatch Riolu
You now have Riolu... after a fashion. Technically, Riolu is still getting ready to be born, which is doubtless one of the most important events in any Pokemon's life.
As with all Eggs, the Riolu Egg is going to hatch after you've taken a specific number of steps. The number of steps necessary actually varies per Pokemon but we believe Riolu will crack its shell and say "hello, world" after approximately 2,000. You can track your progress using the Poketch's pedometer app.
There is a way to speed up the process - have a Pokemon in your party with the Flame Body or Magma Armor ability. The immense heat causes hatching to occur in half the usual amount of time.
How To Evolve Riolu Into Lucario
Riolu's cute but Lucario is deadly. There's probably a small minority of Pokemon BDSP players who prefer the former so very much that they'll keep their Riolu forever and ever, but if that applies to you, you should know something. Your Riolu will never love you.
That's because Riolu evolves into Lucario by maxing its friendship level. This is a hidden statistic in Pokemon games, tied to various parameters such as traveling with a Pokemon, battling with it, feeding it berries and poffins, and avoiding letting it get knocked out during fights. You should also refrain from giving Riolu any of those bitter-tasting herbs. Pokemon hate that stuff.
Having Riolu carry a Soothe Bell will move things along more quickly. One of the maids inside the Pokemon Mansion on Route 212 will hand you this useful little chime free of charge.
Lucario's a force to be reckoned with. A Steel/Fighting-type, it makes for a great addition to your team if a slot's up for grabs. For rougher battles, like the ones with the Elite Four and Sinnoh League Champion, consider having it hold a Life Orb and teaching it Close Combat, Meteor Mash, Swords Dance, and Extreme Speed.
Predictably full of RPGs.