The rival will eventually evolve his Eevee (and thus affect which other Pokemon are in his party) depending on the player's actions early in the game. If the player beats the rival at both Oak's Pokémon Research Laboratory and the early, optional, battle at Route 22, Eevee evolves into Jolteon. If the player loses one of these two battles, or passes up the Route 22 battle, Eevee evolves into Flareon. If the player loses at Oak's Lab, and loses or passes up the Route 22 battle, Eevee evolves into Vaporeon.