How do you play the rival in Pokemon FireRed?
You can only play as the main character, Red.
You cannot get Rayquaza in Pokemon FireRed. Rayquaza is a Pokemon only available in Pokemon Ruby, Sapphire, and Emerald through normal game play and Pokemon Diamond, Pearl, and Platinum by hacking or trading. It is not available throught trade, hacking, or normal game play in Pokemon Fire Red. Rayquaza is a Pokemon of the Hoenn Region (Pokemon Ruby, Sapphire, and Emerald), not the Johto and Kanto regions (Pokemon FireRed and LeafGreen).
If you've chosen Bulbasaur as your starter Pokemon, then your arch Rival has Charizard. He's Professor oak's Grandson, whatever you've chosen to name him. I think there's only one other person in the game who has one of Charizard's early forms, she's a camper and she has a Charmeleon, the Pokemon that will evolve into Charizard at level 36. I've chosen Charizard as my starter Pokemon, and my rival has Blastoise: Your rival always has…