Who are you?

After he knew he was in Slytherin house,he was excited and can't wait to have friends in this Slytherin house.He went to the library to study his book that he bought at Diagon Alley after eating his lunch.
After a few moments,he felt he had stomachache and need to answer the call of nature.So,he quickly ran to boy's lavatory with a stack of books.Alan felt that there is something on the wall.Hence,he stretched his arm to touch the thing and said nervously,"H-Hello,who are you?"He couldn't touch the thing on the wall.
"Pong!"
''Arghh!",Alan shouted on top of his lungs.
"Hey! Hey! Are you okay?" ,his friend asked anxiously.
"O-Oh,it's okay.My books just fallen down from my hand."He smiled bashfully.
Alan heaved a sigh of relief.He rubbed his eyes and stared at the wall again,but this time, the thing had gone. His friend looked at pale face and asked,"What's going on?Are you okay,dude?"
Alan quickly told his friend about the thing that happened just now.So,they decided to check it out in the lavatory on the next day.

ON THE NEXT DAY,Alan and his friend walked into the lavatory without making noise.They were very scared and suddenly saw an invisible things,seems like a ghost at the corner of the lavatory.Their nerves wrecked and can't even move at all.After a few moment,Alan tried his best to asked something to the ghost.
"Who are you?"
After asking "Who are you?",the ghost vanished and disappeared.He and his friend were shocked and hole tightly together.They ran as fast as their feet could carry them out the lavatory and prepared to tell other friends about this situation.