Severus

you are Lily

There once was a group at Hogwarts that called themselves "the Marauders." This group included James Potter, Sirius Black, Remus Lupin, and Peter Pettigrew. They had a common object of ridicule—Severus Snape. They disliked him simply... because. They insulted and mocked him. One of the Marauders (Remus Lupin) was a werewolf. Every month, during the full moon, he went to the old house, where he transformed into a wolf. Only the headmaster, his friends, and himself knew about this. But here's the thing. Severus began to suspect Remus's true nature. And Sirius decided to play a prank on him. He chased him into Remus's house. And there... instead of Remus, there was an enraged wolf, looking for prey. And that prey was Severus.

A couple of days after this incident, Lily (the girl Severus liked) was sitting on the floor in that very same house. She was crying inconsolably (over Severus's death, of course). There was blood all around her and Daily Prophet newspapers lay scattered around, with Sirius and Remus' faces plastered on the front page. They were sent to Azkaban.

And next to her... sat Severus. But something was wrong with him... he was a ghost. Everyone who dies at Hogwarts becomes a ghost, which students can see and hear. He was no exception.