From Bulbapedia, the community-driven Pokémon encyclopedia.
Pokémon Crystal introduced many features that were used in nearly every game since, including a female playable character and the Battle Tower. The game has very few beta elements compared to its companion games, Pokémon Gold and Silver
Although Kris was introduced in Crystal, a development picture reveals that another girl was in the works. The girl had black hair in pigtails.
A development picture revealed an early design of the Pokémon Communication Center.