Pokemon X and Y can be considered as the evolution of the pokemon series, they are smarter, faster, and bigger than their predecessors. The improved graphics and their multiplayer playing experience made the transition to new gen of games beautifully.
With such technological advancement, it is no surprise that the developers at Game Freak decided to go beyond the horizon in providing what they do best; only delivered in a different manner this time.
This is easily one of the creepiest moments in the pokemon series after the story of Lavender Town; the inclusion of a female Hex Maniac character who resides in Lumiose City
Her first creepy appearance is shown when you enter Lumiose City's North Boulevard, in the second floor of Hotel Richissime. Your first sighting of her is rather normal, with her telling you not to disturb her. But when you make your way to the second floor of the unnamed building down the street from the hotel, there prompts a cutscene where your character is frozen with her walking around you saying:
No, you are not the one...
And then disappears.
She also appears in the 2014 Pokemon Omega Ruby and Alpha Sapphire series in a similar manner.
Before your fight with Phoebe of the Elite 4.
Gif from Kotaku
Notice after the frame darkens, the chair on the left is seemingly occupied by a "guest".
Gamers and players alike have theorized that "the one" seems to indicate a specific player ID. Since every player starts with a different player ID, so it's possible that The Ghost Girl has been waiting for a player with the right player ID. However, it is also entirely possible that the player with the right ID has never spoken with her, leaving the whole mystery unsolved.
There are also speculations that The Ghost Girl is a dead relative of Phoebe's, some go to the extent of saying her dead daughter even. There are also speculations involving the new legendary Pokemon, Hoopa who was initially revealed thanks to hackers who made their way into the inner workings of the game.
Players suggest that the one she is looking for could be a pokemon; Hoopa afterall.