Which game was paired with Pokémon Red for the Japanese launch?

Pokémon Green was paired with Pokémon Red for the Japanese launch.

Pocket Monsters Red and Pocket Monsters Green were released for the Game Boy in Japan on 27 February 1996. They were the first two Pokémon role-playing games and established the series' basic structure: exploring a region, catching Pokémon, training them, battling other Trainers, and completing a Pokédex.

Many international players know the first generation through Pokémon Red and Blue. Outside Japan, Nintendo and Game Freak used Blue as the companion version instead of Green. The international Blue version also received graphical and technical changes compared with the original Japanese releases.

The paired-version model encouraged trading. Certain Pokémon appeared in only one version, so players needed to connect their games or trade with friends to complete the Pokédex. This design made the Link Cable central to the franchise's social identity.

Pokémon Yellow followed in Japan in 1998. It was inspired by the anime and gave players a Pikachu that followed the protagonist rather than beginning with a choice among the original three starter Pokémon.