He couldn't sleep. The one he closed his eyes and laid his head on his pillow he heard her voice whispering to him, calling out to him.
"What did I ever do to you?"
"You killed me. You did it!"
"It's your fault! Yours!"
Not able to stand the whispers any longer, he would open his eyes night after night and be met with her translucent image before him. He was losing his mind. And that was why he found himself in Professor Dumbledore's office in the middle of the night.
"She's dead, Professor; it's my fault."
"Sirius, you've said yourself that— "
"I drove her to it! I started the rumor! That's why she—…it's because of what everyone was saying about her but it was a lie!"
"Your guilt does not make you—"
"No! She would not have just killed herself! I would have noticed the signs! I would have known!"
"Sirius, it's probable that Miss Blyth had her own secrets—"
"No! It's my fault! That's why I keep seeing her. That's why I'm losing my damn mind."
Dumbledore hesitated, "Sirius, I believe you should know that Miss Blyth remains at the school as a ghost."