Jeff was a 13-year-old who moved to a new neighborhood with his brother Liu and their parents. On their first day of school, they were accosted by 3 bullies named Randy, Keith & Troy, who Jeff brutally beat up after they threatened them with knives. Liu took the blame for it and was arrested, sending Jeff into a deep depression. The bullies later attack Jeff again at a party, and Jeff kills one of them, but not before getting covered in bleach and lit on fire by Keith. During this fight, Jeff's mind permanently snapped.
When Jeff woke up, he was at the hospital and his head was wrapped in bandages. When the bandages were taken off, it was revealed that his face had become disfigured and ghastly pale from the burns, with bright red lips and a leathery texture. Jeff said that he liked his face this way and laughed hysterically, but the doctor foolishly thought that this insanity was merely a side effect of the painkillers and let Jeff go home.
Later that night, Jeff's mother finds him carving his face into a permanent smile so that he would no longer have to exert energy to smile, as he put it, and burning off his eyelids so he can always see his face. Jeff's mother went to her husband to tell him that their son had gone completely mad and needed to be killed, but Jeff caught them and stabbed them to death. This woke up Liu (who was recently released), with Jeff telling him as he was about to plunge the knife into his body, "Go to sleep".
Jeff then goes on a serial killing rampage killing those who refuse to sleep at night, and as the story continues, Jeff renamed himself as - "Jeff the Killer".
Personally:
Before being burnt alive, he was a quiet and fairly anti-social teenager, and not that people actually knew him very well. But shortly after the incident, he became torturous, aggressive, bloodthirsty, and violent, making him one of the most dangerous serial killers in his home town.
Jeff is widely known for luring his victims to an eternal slumber, using a kitchen knife and his soft, eerie tone of voice. Even though he prefers to murder his victims with knives, he is more than willing to use any form of w eapon when placed in a desperate situation. He is even an extremely stealthy and mischievous individual, able to break into victim's houses almost always without getting himself caught in the act.
