Those that have started their adventure in Pokémon Let’s Go, Pikachu! and Pokémon Let’s Go, Eevee! have been confused about an unexplained green icon that nearly constantly appears on-screen.
There is a simple explanation behind it, in that Game Freak has implemented the green icon to indicate when a Support Trainer can join in.
Generally speaking, this is possible while you are exploring the Kanto region and before you prepare to make your next move in battle. The green icon will disappear when it is unavailable to be activated, such as when a cutscene is happening or as your Pokémon is striking an opponent’s pocket monster with an attack.
This Support Play feature has been introduced to let two players explore the world of Pokémon together, with the chance to catch pocket monsters by throwing Poké Balls at the same time or teaming up to battle competitive Trainers.
For a second player to join, you simply need to shake a Joy-Con (or Poké Ball Plus) and they will warp alongside the main character, and will always be the opposite gender to whichever was originally selected.