From Bulbapedia, the community-driven Pokémon encyclopedia.
Silver is the rival of Gold and Kris, and is about their age. He has long red hair and piercing eyes.
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.
Like Brendan, he played no role in the anime, despite being a main character in the games. Similarly, his only appearance in anime is a brief cameo, in the original Japanese opening of The Legend of Thunder (Aratanaru Chikai), where he was shown with his Nidoking.
In the game
Silver's first (and currently only) appearance in the Pokémon video games took place during the second generation. He was first seen lurking near a window to the Elm Pokémon Laboratory, and he later broke in, stealing a Pokémon (commonly thought to be a Totodile, though this is unproven). He later used this Pokémon to battle with both Gold and Kris, doing this many times as they journeyed through Johto.
Throughout his travels, Silver showed a strong hate for Team Rocket. He strongly interfered with the revived Team Rocket's plans at least twice, including their experiments with radio signals at the Lake of Rage. His stated reason for this dislike was that they are "weak" when cornered, and that he only cares about strong opponents.
Shortly after Gold/Kris defeated the Pokémon League, Silver took up training in the Dragon's Den. His personality has been likened by the Dragon Master to be similar to Clair's.
The starter Pokémon that Silver has will depend on which Pokémon the player chose to bring on Professor Elm's errand at the beginning of the game. As with all rivals, it will be the Pokémon whose type weakens that of the player's starter.
Silver is the player's final opponent in the game, after completing every other battle. After his defeat, players will move on to Round 2, where all of the battles are much harder, or have beaten the entirety of the game. He is named just "Rival" in this game.
Silver has the following Pokémon in this game: