Red
Red is known throughout the Pokémon world as the Champion from Pallet Town, as well as a living legend for his defeat of Team Rocket in Kanto during his quest. He is the final opponent in the Generation II games Pokémon Gold, Silver, and Crystal, their Generation IV remakes Pokémon HeartGold and SoulSilver, and Pokémon Stadium 2. He is a participant in the Pokémon World Tournament's Champions Tournament in Pokémon Black 2 and White 2 and works as a boss of the Battle Tree alongside Blue in the Generation VII games Sun, Moon, Ultra Sun, and Ultra Moon. In Pokémon: Let's Go, Pikachu! and Let's Go, Eevee!, he appears as the highest-level Trainer in the game, capable of being challenged after defeating six Master Trainers.
Blue
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. Blue does not specialize in a particular type of Pokémon and 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.
Green
Green appears in Pokémon: Let's Go, Pikachu! and Let's Go, Eevee!. She is first mentioned by Trace after the player has become Champion as a girl looking to catch a powerful Pokémon spotted in Cerulean Cave. After the player has caught Mewtwo in Cerulean Cave and exited the cave, Trace will appear again and mention how the girl looking for Mewtwo had entered the cave earlier. Green can then indeed be found inside Cerulean Cave, at the same spot where Mewtwo was located earlier. When first interacted with, she will toss a Poké Ball at the player, having mistaken them for a Pokémon. Upon learning that the player managed to catch Mewtwo before her, she immediately challenges them to a battle. After being defeated, she will hand the player both of Mewtwo's Mega Stones: Mewtwonite X and Mewtwonite Y. She will then ask the player to become one of her Pokémon, along with Mewtwo, and throws several Poké Balls at them, attempting to catch them like a Pokémon.