The twins:

Lost their parents soon after birth and grow up in an orphanage. At the age of the eight they got thrown out and lived at the streets. They got caught by some gangster and had to work for money. HongKi (one of the twin) find a way to escape from the gangster and also found a secure place for his sister live. But she had to act as a boy from then on, it was the only way for her to stay be safe. Before he left her alone he gave her a necklace and promised he would come and find her for sure.

From them on Park ShinHye is obediently waiting for Lee HongKi to come and get her.

Kay:

Key is a girl who has to hide her gender. Since the age of nine he lives with his parents who are always away on some business trips. He has two childhood friends Shim ChangMin and Kim JaeJoong. He grow up with them, those three are like family.

ChangMin and Kay attend the same school, they are always together and are rather popular with girls. Some girls even made rumors about them being a gay coupe since they always refuse confessions. ChangMin like to listen to the rumors and tease Kay with them.

Because of an accident that happened when he was fourteen he, he had to reveal his gender to his friends. It was a big relieve that his friends didn't leave her after hearing his story.

Shin ChangMin:

Ever since he grow up he got everything he wanted, but he never felt love. His parents are rich and are never home. If they are home they always talk about work, he feels always alone and lost.

After getting to know Kay and JaeJoong he felt like he f