Ko Do-Young is a dangerous cold-blooded killer who stalks his victims to the end, while hiding behind his guise graduated from college as a laboratory assistant in the scientific industry, grew up in a poor family and always tried to provide for his little sister Regina, since he is her older brother. He is always sweet and gives all his love to his sister because she is the most precious thing in his life. Do-Yeong has no mercy for those who harm his little sister,he likes to call her 'button'