Ahn Doo-hoon is main villain of Korean series Secret.
Five Years ago
Yoo-jung's father learns the truth about his daughter's sacrifice and comforted Doo-hoon. His mother put fake death certificate on her own grandson and sends him for adoption, which left Yoo-jung believing that his son dead.
Five Years Later
Doo-hoon admit to Min Hyuk that he killed Ji-hee. Yoo-jung became angry and slaps Doo-hoon for made her misery (her father's death and their son living with another family) later his mother was arrested for fraud.
Doo-hoon apologized and cried for Yoo-jung for his greed and lost of huminty as a father as Yoo-jung left.
Doo-hoon is found guilty for Ji-hee's death and put Yoo-jung into jail.
After Yoo-jung won custody of San, Doo-hoon reviced mall from Se-yeon.