What major character option did Pokémon Crystal add to the main series?

Pokémon Crystal added the option to play as a female protagonist.

Released in Japan in 2000, Pokémon Crystal was an enhanced version of Pokémon Gold and Silver. Earlier main-series games had used a fixed player character, but Crystal allowed the player to choose between a male and female trainer at the beginning of the adventure.

The female character is commonly called Kris in later materials, although the game itself presents the choice through its player-character selection. The option did not create a separate story campaign; it changed the playable trainer’s appearance and some associated presentation details.

Crystal also introduced animated Pokémon sprites during battles, additional story material, and the Battle Tower. It remained a Game Boy Color title and served as the final main-series game of the second generation. The female-player option later became a standard feature across the core series.