Riley's first appearance in Diamond, Pearl, and Platinum is at Iron Island. He helps the player to get through the deep mines on the island, and assists in the defeat of some Team Galacticgrunts. After the Grunts have been defeated, Riley offers the player a Pokémon Egg that eventually hatches into a Riolu. In Platinum, he will give the player HM04 (Strength) before the player enters Iron Island's cave.
Apparently, Byron had previously asked Riley to become a Gym Leader, though Riley politely refused, saying that Byron's son Roark should be considered first.
Riley mentioned to the Team Galactic members that Iron Island actually belonged to him, suggesting he may be the owner of the mines and someone else reopened the island as a place for wild Pokémon to live as well as a place for people to train.