From Bulbapedia, the community-driven Pokémon encyclopedia.
Silver is the rival of Gold and Kris, and is about their age. He has long hair and piercing eyes.
Silver steals a Pokémon from Professor Elm. At least one other Pokémon of his was stolen from another trainer. He travels Johto, meddling with Team Rocket's plans to call back their once leader Giovanni. Silver hates weak trainers, and has vowed to become strong and wipe out the weak trainers. This goes even for Team Rocket, as the trainers alone are weak.
Fan speculation is that Silver may be Giovanni's red-haired child, due to Pokémon FireRed and LeafGreen's Fame Checker's many references to Pokémon Gold and Silver versions, and to Silver's dark demeanor and detestment of Team Rocket.