DISCLAIMER: This is merely a creepypasta, not the actual explanation of the ghost girl, this is based off a theory which you can find here. Edit
It was a normal day in Lumiose City. Sarah had just woken up and she went for breakfast and got changed. After that, Sarah went downstairs and said goodbye to her mother. "Maybe I will visit that hotel one day..." Her mother mumbled to herself. "What?" Sarah asked her mother. "Nothing.... Where are you going anyway?"
"Just to the train station to pick up Johnny!" Sarah replied. "Ok, be back by lunchtime!" Said her mother.
As she left the house, her Holo-Caster made a beeping noise, indicating that she had a message. When Sarah activated the device, there was a special news announcement. "There has been multiple murders across the Kalos region, it is said that it was the same murderer that kill Tim Smith 3 years ago." Said the news reporter. Sarah's dad was Tim Smith....he was murdered in a hotel. Sarah felt bad for the victims and stood still for a minute.
Sarah went to the train station. Her brother,Johnny got off the train but a man came off the train holding a knife to his neck. Sarah screamed and quickly grabbed Johnny and took him behind one of the boards. She wrote on it in red pen to say something to Johnny without being heard. The message said "I'm going to go for help. Wait in the usual place." Sarah then ran off and the man quickly ran after her. Johnny went to the 2nd platform to wait for his sister to return.
Sarah ran to the police station to get help but before she was able to reach it the man grabbed her by the hair and pulled her to one of the buildings. Sarah was taken to the elevator. The man dragged her into the elevator and pressed the button to go to the second floor, he had left her feet hanging out the elevator door and when the doors closed......her feet were chopped off..... Sarah screamed in agony as the elevator went up. When the elevator made it to the 2nd floor, the man dragged Sarah out of the elevator,slit her throat, shot her three times in the stomach and left her to die..... Her body was found 3 days later and Johnny never returned to his house....the man had killed him too... He still lurks around the station because he doesn't know that he is dead.
Sarah now waits in the very building she died in to have her brother meet her. When you see the ghost girl in lumiose....she will flee to a hotel without her brother, when you encounter her in there....she meets with her father who is an invisible ghost....and passes over to the afterlife......without her brother...