[This is a brief recap of the backstory for those of you who haven't read the manga]

Hyun-Bin hadn't always been Hyun-Bin. He hadn't always been a he.

Years ago, when Hyun-Bin was only five or six, she had gone by a different name - Frog - while concealing her identity as a female in order to survive, along with her younger brother, Pig. The two of them, orphans both, patrolled the trains every day, begging for money to turn in to their handlers. If they failed to make enough money, they were beaten or worse.

One of such times, Frog and Pig were locked outside in the dead of winter, and Pig slowly grew colder and colder, moving less and less, until he stopped moving entirely.

Then there was no more Pig, just Frog.

When the opportunity came her way to abandon her life as a beggar and train to become the bodyguard for a wealthy businessman's granddaughter, she agreed.

There was one condition, however: she had to disguise herself as a boy.

From then on, she was forced to scream in order to ruin her vocal chords so nobody would be able to detect her gender, train hard in martial arts styles, and forsake any emotional attachments she had.

Time passed, and she eventually was deemed worthy of fulfilling her job: guarding Yae-Ha, the doll-like granddaughter of a rich corporate owner. Yae-Ha was kind and brave, but she didn't truly understand how the world worked, and she could only guess about how Hyun-Bin really felt, not to mention that she believed Hyun-Bin to be male.

Nobody knew the truth about Hyun-Bin with the exception of Yae-Ha's grandfather and several members of his staff.

Everything began to get complicated when Yae-Ha entered high school, Hyun-Bin at her side as always, and Yae-Ha fell head-over-heels for Lee Yun-Ha, the leader of the school gang who had been responsible for a car crash that had killed three people and left him with a heart that refused to open to others.

One way or another, he and Hyun-Bin eventually became friends, and Hyun-Bin continually tried to push Yun-Ha towards Yae-Ha as per instructions.

Hyun-Bin's heart was breaking, though, because he loved Yun-Ha too.