The "Lumiose City Ghost" legend is an urban legend revolving around the 2013 role-playing games Pokémon X and Y.
Description
Entering the second floor of a certain building in Lumiose City will cause the music to stop and the lights to flash, followed by a ghostly girl materializing behind the player and gliding towards the bottom of the screen in a frozen walking animation. She will then mutter "No, you're not the one" before floating offscreen and disappearing completely.
Theories began to spread about her role. One hypothesis said that she would only talk to a player with a certain trainer ID, and another said that she was the key to unlock some ultra-rare Ghost-type legendary. Currently, none of the prevailing theories have any real evidence to support them; it appears that the ghost girl is merely an easter egg.