Reece Fleming, 21, pleaded guilty Friday, The Scotsman reported. He told the High Court in Edinburgh he had drunk 14 bottles of beer before the attack, which occurred after he watched television with his 18-year-old victim and her mother.
The victim said Fleming pulled out a knife, stabbed the couch and told her to undress.
"No words can explain how I felt that night. Petrified would probably be the best I can manage. I was more scared that something would happen to my mum than me," the victim said. "He had watched the movie 'Paranormal Activity 4' the week before. At first I thought he was ... joking around. It was when he stabbed the couch that I realized he was being serious."
She said Fleming told her he was "David" and that "Reece" had gone. After the attack, he fell back on a bed and then, a few minutes later, opened his eyes and asked where he was.
He begged her not to tell "Reece" what he had done, she said.
Fleming, a delivery driver for takeout food, was jailed immediately after his guilty plea. He is to be sentenced next month following a psychiatric evaluation.