"Congratulations! The feelings you have for your Pokémon... And the Pokémon that responded to those feelings with all their might... They came together as one, and created an even greater power. And thus, you were able to grasp victory today! You are rightfully the Hoenn region's new...(interrupted by rival)"
Steven is the son of Mr. Stone, the creator of the PokéNav. Like Cynthia in the Generation IV games, Steven is found throughout the game, giving items and tips to the player throughout the game without revealing their status as Champion.
Pokémon Emerald
Steven in the anime
Unlike in Pokémon Ruby/Sapphire, Steven Stone is not the Champion of the Pokémon League in the Emerald Version. He is found in the Meteor Falls after you became the Champion of the Pokémon League for the first time. You can battle with Steven there, but he is much stronger then in Ruby/Sapphire. He is merciless and has the uses the same Pokémon. You can only battle him once, unlike in Ruby and Sapphire. Also, the level of his Pokémon has increased by the 20s compared to the Ruby and Sapphire versions of the game, and even more compared to the team tag battle Pokémon he used to face Team Magma.
Pokemon HeartGold and SoulSilver
Steven Stone in Generation III
In Pokemon HeartGold and SoulSilver, Steven is found in the Silph Co. building after you defeat Red. He asks you which stone you like, then gives you one of the Hoenn starters depending on the color of stone that you chose. If you pick the red stone, he gives you Torchic. If you pick the blue stone, he gives you Mudkip. If you pick the green stone, he gives you Treecko.