Riolu will not appear in Pokémon Mystery Dungeon: Explorers of Darkness until a rescue mission involving it has been completed, using a Wonder Mail code such as the following:

Riolu Sitting Cuties Plush - 5 ¼ In. | Pokémon Center Riolu Sitting Cuties Plush - 5 ¼ In. | Pokémon Center

Moves marked with an asterisk (*) must be chain bred onto Riolu or copied using Mirror Herb in Generation IXIt can discern the physical and emotional states of people, Pokémon, and other natural things from the shape of their aura waves. It's tough enough to run right through the night, and it's also a hard worker, but it's still just a youngster. Riolu came to the Haunted Zone to explore. He can't resist the fun of playing Tangrowth's Swing-Along. Absol's Hurdle Bounce is another favorite.

Riolu Plush | Pokémon Center Official Site Riolu Plush | Pokémon Center Official Site

An Emanation Pokémon. It constantly emits Aura energy. When it's frightened or sad, this energy grows stronger and informs its companions of its situation. It's known for its very flexible yet surpassingly powerful body. Thanks to its strength, it can crest three mountains and cross two canyons in a single night. When it levels up, it evolves into Lucario." Game data Pokédex entries This Pokémon was unavailable prior to Generation IV. It knows how people and Pokémon feel by looking at their auras. It doesn't approach dangerous opponents.It has the peculiar power of being able to see emotions, such as joy and rage, in the form of waves. In Vexing Vespiquen & Unmanageable Mothim II, Diamond helped protect a Riolu Egg from two Galactic-hired thugs. After they were defeated, it hatched and decided to stay with Riley. When Riolu became a starter in Pokémon Mystery Dungeon: Explorers of Sky it became the first starter Pokémon to be unable to be recruited or captured in another game of its trio as it cannot be found in Explorers of Darkness without using Wonder Mail.