A lot of Hajun's history is unknown. It is known that Hajun has some sort of connection with North Gangbuk High school as he had the number of the Top Dog and had fought with Daniel in the past. At one point, he took over West Gangbuk High and became the top dog using his boxing skills. It is also known Hajun reigned at the top at one point, almost being unbeaten, but lost his desperation. Jaeha and Daniel have collectively said that Gangbuk would be turned upside down if Hajun became desperate.