Oh Se Hun realized the mistakes he had made towards her and he was more than sorry. 

Lee Ha Na was just an ordinary worker at his massive company but he let her struggle just to finish her report. He made her life upside down.

Just because he reminded her of his disastrous ex-girlfiend.

When Ha Na decided to leave, Se Hun realized that he fell for her. A little too late and he let her go.

Just like that ..?

Regret takes over but he is discouraged by the situation. He couldn't face her. 

He decided to write a letter.