Jeon Nam San and Jeon Yeo Mun, the identical twins but unfortunately their healths were not.
Namsan had a dream to be a star, to make his parents proud.
Yeomun had no dream, she only went with the flow, let her fate took her somewhere-- anywhere.
But her brother fell ill and asked his sister to replace his place for a year or two. She agreed to take his place for a while since she had no job whatsoever, plus they also needed money.
Faking identity for a year or two shouldn't be that hard anyway... right?