An assassin.

Someone who executes a person for political or religious reasons.

A normal school boy.
The one who sat in the far back corner of the classroom.

Who would've thought that he was so deadly and dangerous?
Who would've thought, Oh Sehun was part of an assassination?

No one was suppose to know about Sehun's hidden identity. No one. But Ji-Hyun found out. She found out that Sehun was an assassin. Ji-Hyun, who was walking home one day after school, over heard someone's conversation on the phone. The voice, it belonged to Sehun. He hid in the alley so no one could eavesdrop on him, but Ji-Hyun heard every last word that left his mouth.
His mission and his job, was to end someone's life?
In order to keep his identity a secret, Sehun had to execute Ji-Hyun before someone knew about him. Ji-Hyun pleaded that she wouldn't tell a single soul. Sehun didn't believe her. But the look in her eyes told Sehun otherwise. Though if she didn't keep her promise and told someone about Sehun's identity, he would kill her without a second thought. 
Sehun could get away with anything, especially killing Ji-Hyun. He worked under the government and his job was to assasinate those he was assigned to.

He hid in disguise as a normal high school boy attending the same school as Ji-Hyun. He never would have thought he would fall for someone, let alone it being the very person who he had wanted to execute...

...Ji-Hyun