From Bulbapedia, the community-driven Pokémon encyclopedia.
Pokémon XD: Gale of Darkness (Japanese: ポケモンXD 闇の旋風ダーク・ルギア, Pokémon XD: Whirlwinds of Darkness, Shadow Lugia) is the title of the new Nintendo GameCube game to be released August 4th, 2005 Japan and some time in October in North America. The "XD" in the game's title stands for "Xtra Dimension." The game, being developed by Genius Sonority, is reported to be a full RPG similar to that of the Game Boy Advance games. Not much else is known about the game at the current time aside from the main characters and screenshots that depict Pokémon Colosseum 3D character models.
It is as of yet unknown if Pokémon XD will include more new Pokémon than Munchlax, which is currently the only known new Pokémon appearing in Pokémon XD.
Currently there are three main characters: the Main Character (this is his current name), his little sister, Mana and Shadow Lugia, which seems to be very important in this game, as it is mentioned in the game's title.
The Main Character has a Snag Machine like Pokémon Colosseum's Hero Wes and an Aura Searcher, which makes the Dark Auras of Shadow Pokémon visible similar to Pokémon Colosseum Sidekick Rui's ability to see the Dark Auras.