He is the Hoenn Champion. But he isn't in Soul Silver for a battle. He'll tell you that Latios is roaming Kanto. He let's you have one of the Hoenn Starter Pokemon. He also let's you trade with him.
Steven isn't battleable in the game.
You can't battle Steven in Pokemon SoulSilver or HeartGold.
In Saffron City
you cant beat steven in pokemon soul silver only in emerald,ruby and sapphire
steven has to be found in the museoum in peweter city an trade pokemon with him at saffron city
Pewter museum!! :D
to find meteorites and crystals.
Unfortunately, you cannot battle Steven on Pokemon HeartGold or SoulSilver unless an event comes in the future.
You can get latios after you meet Steven, and latias through the enigma stone event.
He is in Silph Co. but you need to trade rotom over from Pokemon platinum
You don't find Steven in Mount Silver. You find Steven in Silph Co. after beating Red. He will give you a Hoenn starter.
After you get your Hoenn starter and speak to Steven in the Pewter Museum, Steven will be in the Silph Co. bottom floor. He has a Beldum with a Dawn Stone to trade for a Forretress.