A new female trainer to the video game series appears in Pokemon Let’s Go Pikachu and Let’s Go Eevee. She is based off the female character that appeared in early concept art of Red and Green, and also appeared in the Pokemon Special manga.
You can find her in the Cerulean Cave after capturing Mewtwo. She will be waiting in the same spot as Mewtwo. After the battle, she will disappear until you defeat the upgraded Pokemon League with the Elite Four and your Rival. Green can then be found standing in the middle of Cerulean City.