Steven Stone is the son of the president of Devon Corporation, and is a collector of rare stones. he first meets the protagonist ( Brendan or May) in Dewford Cave, and later interacts with the player just before reaching Route 119, and after the rise of the legendary duo Kyogre & Groudon.
In Pokemon Ruby/Sapphire, he finally confronts the player as the Hoenn Champion, and must be defeated to complete the story.
In Pokemon Emerald, he plays a more active role in the story, helping the player thwart Team Magma at the Mosdeep Space Centre. he also gives up the role of Hoenn Champion, giving it to former Gym Leader Wallace. he is still available for battle at the deepest area of Meteor Falls after the main storyline, with the same team, but all members 20 levels higher.
Steven gives the player a Beldum after completing the story by visiting his house in Mosdeep City, and taking the Poke ball on the table.
In HeartGold and SoulSilver, Steven appears after the player defeats Red, giving them one of the Hoenn starters, and later offering to trade a beldum for a Forretress.
Pokemon Ruby & Sapphire(as Hoenn Champion)
Skarmory - Lv. 57
Cradily - Lv. 56
Claydol - Lv. 55
Armaldo - Lv. 56
Aggron - Lv. 56
Metagross - Lv. 58
Pokemon Emerald (in Meteor Falls)
Skarmory - Lv. 77
Cradily - Lv. 76
Claydol - Lv. 75
Armaldo - Lv. 76
Aggron - Lv. 76
Metagross - Lv. 78