Blue Oak (Japanese: グリーン Green) is the rival of the player in the Generation I games, as well as in Pokémon FireRed and LeafGreen, their Generation III remakes. By the end of the main game in these games, he also becomes the Pokémon Champion of the Indigo Plateau.

Sometime during the three years between the plotline of the Generation I games, Pokémon FireRed and LeafGreen, and the events of the Generation II games and their remakes, Pokémon HeartGold and SoulSilver, as well as during the events of Pokémon: Let's Go, Pikachu! and Let's Go, Eevee!, Blue takes the place of Giovanni as the Viridian City Gym Leader, as Giovanni had disappeared after he disbanded Team Rocket. Blue does not specialize in a particular type of Pokémon and, like Giovanni before him, he gives anyone who defeats him an Earth Badge.

In Pokémon Sun, Moon, Ultra Sun, and Ultra Moon, he is one of the leaders of Alola's Battle Tree, the other being Red.

His voice should be similar to his voice in Pokémon Origins where he was voice by Lucien Dodge. That or something similar to Roger Craig Smith's Sonic.