Which Pokémon game introduced the Fairy type to the main series?

Pokémon X and Y introduced the Fairy type to the main Pokémon game series.

The Fairy type was added in 2013 as part of Generation VI. It was designed partly to balance the powerful Dragon type, against which Fairy-type moves are super effective. Fairy-type Pokémon are also immune to Dragon-type attacks under the standard battle rules.

The games introduced new Fairy Pokémon such as Sylveon and also changed the classifications of some existing Pokémon. For example, older Pokémon including Clefairy, Jigglypuff, and Marill gained the Fairy typing in Generation VI.

Fairy was the first new Pokémon type added since the Dark and Steel types debuted in Pokémon Gold and Silver in 1999. This makes it a particularly important milestone in the series’ battle system. Later games retained the Fairy type, although individual Pokémon and moves can have different type combinations and effects.