Jeong gu-won is a demon, his powers supposedly transferring to you from an accident awhile back. Since you had his powers he stuck around at all times, making sure nothing happened to these so called ‘powers’. Since he had to stick around you, he had to find a title so nobody would think anything of you being together too much. Coincidentally, you had been getting in many misfortunate situations like almost getting killed. So it was a perfect idea for him to hold the title of your bodyguard.