From Bulbapedia, the community-driven Pokémon encyclopedia.
The Phantom Cards are two cards from the Pokémon Trading Card Game and Pokémon Card GB2: Here Comes Team GR! for Game Boy. They can only be obtained through Card Pop!. Because a player can only Card Pop! with the same person once, these cards are considered the hardest cards to get in the game. There are two Phantom Cards: Venusaur (which can be obtained as an actual card with the purchase of the game's Player's Guide) and Mew. Both of these cards are needed for the Mysterious Pokémon Deck.
In the second game, the two Phantom Cards are Here Comes Team Rocket! and Lugia; in addition, Mark/Mint can buy the Phantom Cards from the first game in the Game Center on GR Island.