The Pokémon Let's Go games have been promoted as loose remakes of Pokémon Yellow, down to including Pikachu as a starter and battling Team Rocket from the anime throughout the adventure.
The latest trailer for Pokémon Let's Go Pikachu & Let's Go Eevee showed us the eight Gym Leaders and the members of the Elite Four, along with a few surprising characters...
The end of the trailer shows an encounter with Blue (the rival from Pokémon Red & Blue who formed the basis for Gary Oak in the anime), Red (the protagonist of Pokémon Red & Blue who also appeared as an incredibly tough post-game boss in Pokémon Gold & Silver) and a girl who appears to be Leaf from Pokémon FireRed & LeafGreen, except that her design is closer to Green from the Pokémon Adventures manga.
Blue is shown hanging outside a Pokémon Center and appears to be talking to the character, which means that he potentially will only be an NPC. There is also the possibility that you will have the chance to battle him, where he will likely use the team that allowed him to become the Champion in the original games.
Red is shown as part of a battle intro, so we know for a fact that you will have the chance to battle him in the game. The question now is whether his team will be as strong as his Pokémon Gold & Silver encounter or whether it will be scaled down.
The appearance of Leaf is the most puzzling of all. Leaf was the female protagonist of Pokémon FireRed & LeafGreen, but she was actually planned to appear in Pokémon Red & Green, as she appears in several pieces of concept art for the game.
The development of Pokémon Red & Green was notoriously troubled, which is why the developers weren't able to include a female playable character until Pokémon Crystal.
Green did make a prominent appearance in the Pokémon Adventures manga, where she stole Professor Oak's final starter Pokémon and goes on her own journey. Professor Oak defeats Green in the semifinals of the Pokémon League tournament, but he recognizes her skills and officially bequeaths his Squirtle to her.
The resemblance of Leaf to Green seems to be a reference to her original design in the concept art of Pokémon Red & Green. We aren't sure exactly what role she will play in the game, but it's possible that she will be a third post-game boss to rival that of Blue and Red.
Pokémon Let's Go Pikachu & Let's Go Eevee will be released on November 16th.