Walkthrough
After catching the Legendary Pokemon Mewtwo and leaving Cerulean Cave, your Rival will again appear. He will mention how another trainer went into Cerulean Cave hoping to capture Mewtwo, stating she is probably still within the cave. By going back into Cerulean Cave, you will eventually come to find this trainer. When trying to speak to the her, this trainer will accidentally throw a Poke Ball at you, thinking you were a Pokemon that was trying to sneak up on her.
After apologizing, she will introduce herself as Green. Believe it or not, she was key character back in the First Generation Pokemon Manga. She will mention how she is there trying to capture Mewtwo, and then she will realize that you already caught it. Frustrated, she will challenge you to a battle. After defeating her, Green will start appearing outside the Bike Shop in Cerulean City. She can be battled again each and every time the Elite Four has been defeated in a rematch.
Pokemon Trainer Green
Level 66 Clefable: Moonblast, Light Screen, Reflect, Tri Attack
Level 66 Gengar: Dark Pulse, Shadow Ball, Sludge Bomb, Will-O-Wisp
Level 66 Kangaskhan: Dizzy Punch, Sucker Punch, Thunder Punch, Brick Break
Level 66 Victreebel: Power Whip, Poison Jab, Sucker Punch, Leech Life
Level 66 Ninetales: Fire Blast, Dark Pulse, Foul Play, Hyper Beam
Level 68 Mega Blastoise: Hydro Pump, Flash Cannon, Ice Beam, Fake Out