The enhanced version of Pokémon Gold and Silver, Pokémon Crystal has you travel across the Johto region once more, with several brand new features that became staples in the Pokémon franchise. This includes the ability to choose your gender, the addition of animated sprites, the Battle Tower, and brand new subplots featuring the mysterious Unown and the fan-favorite legendary dog Suicune.
The game is now available for download for Nintendo 3DS via the Nintendo Eshop, bringing it to a whole new generation of fans in the modern era. This edition also adds some online features like battling and trading.
From the game's Wikipedia page:
"Pokémon Crystal Version is a role-playing video game developed by Game Freak and published by Nintendo for the Game Boy Color. It is an enhanced version of Pokémon Gold and Silver, and is part of the second generation of the Pokémon video game series. It was released in Japan on December 14, 2000, North America on July 29, 2001 and Europe on November 2, 2001.
On January 26, 2018, Pokémon Crystal was re-released worldwide via the Nintendo 3DS Virtual Console.
The gameplay of Pokémon Crystal is largely the same as in Gold and Silver, although it holds several new features. It is the first Pokémon game to allow players to choose the sex of their character, while previously the character was always male. Pokémon have animated sprites; for example, when a Cyndaquil enters battle, the flames on its back flicker. This feature was absent in Pokémon Ruby and Sapphire and Pokémon FireRed and LeafGreen, before reappearing in Pokémon Emerald and all subsequent games. In addition, a couple of subplots were added, one involving the legendary Pokémon Suicune, featured on the front cover of the game, and the other involving the Unown. The game's biggest addition is the Battle Tower, a new building which allows players to participate in Pokémon Stadium-like fights. The Japanese edition of the game was exclusively bundled with the Mobile Adapter GB, a device that allowed for connecting with other players via a mobile phone."