Which Pokémon game introduced double battles to the core series?

Pokémon Ruby and Sapphire introduced double battles to the core Pokémon game series.

Released for the Game Boy Advance, the games allowed two Pokémon on one side to battle simultaneously when the conditions were appropriate. This expanded the usual one-on-one format by making moves, abilities, targeting, and team combinations more important. A move could affect one target, several targets, or both Pokémon on the user’s side, depending on its rules.

The Hoenn games also introduced Pokémon Abilities and natures, which gave individual Pokémon additional passive traits and statistical tendencies. Together with double battles, those systems made team construction more strategically varied than in earlier generations.

Double battles are not the same as multiplayer battles involving two human players. They describe the number of Pokémon active for each side during a battle. Later games added other formats, including triple and rotation battles, but Ruby and Sapphire established the core double-battle format.