From Bulbapedia, the community-driven Pokémon encyclopedia.
Twins (Japanese: ふたごちゃん Futago-chan, Twins) are a pair of Pokémon trainers that first debuted in the Generation II games. They are two identical young girls. In Generation II, they fought in regular Pokémon Battles. In Generation III, they fight in Double Battles.
They use Pokémon that complement eachother, such as Plusle and Minun or and Ledyba and Spinarak.