Kim Jong Kook, 35 years old. Youngest of 2 siblings. Successful business man. Workaholic. Love never been in his priority list for the last 10 years until he met her. She loves him at the first sight but he needs to fight for her affection with her "big brother".

Song Ji Hyo, 29 years old. The only heiress of wealthy business man. She believes love at the first sight when she first met him. She adores her "big brother" so much, love him as little sister should do. She knows he love her the same way too. But she hate when her big brother being loved by other woman than her. She does anything to spoil the relationship. Little she knows that her big brother love her more than a sister but he is restricted by promises that he could not break.

Kang Gary, 33 years old. He is secretly in love with his "little sister" and struggling to push away the forbidden feeling. He is opening his heart to look for love other than her but she spoilt it everything it happens. He doesn't want her marriage ruins because of her affection as over protective little sister take over her love to her first love and husband.