"S" is a known bully caretaker, who avenge her bullied clients – for price, of course.
"S" is a nerd named Song Ih Kang, with a twin brother Ah Eul Kang, whom she love anything. Ah Eul was treated as a prince while Song Ih was treated as cursed child from their parent. When they were young, Song Ih used to hate him because Ah Eul geta everything from his family but he apologize her for getting everything. That's when two them started strong bond together.
When Song Ih learn that Eul Kang attempt to suicide, she promise she'll avenge someone from his school called Chungha High School which is expensive and all boy school. She attend and pretend to be Ah Eul Kong to find the reason why Ah Eul was driven to suicide, including mysterious handsome but "bad" guy named Seo Ga Yoon.