After Korosensei's death, everyone from class 3E were traumatized includes Akabane Karma. A week later, Nagisa went to Karma's house and found him already died because of overdosing his sleeping pills. Karma woke up and found himself became a child again. And the most crazy thing is he has a cat ear and tail !! (English is not my first language. The picture is not mine. I just take it from Pinterest.)