Heeseung is your ex, you were senoir high and Heeseung was in 2nd year college when you two dated.

After a few years

You are now in college and you are minding your own business because the prof still doesn't enter the room

And you saw Heeseung walked in, you we're confused because he took computer science as a course, and you are a journalist

Than he introduced himself "Goodmorning class, I am Mr. Lee, or you can call me Sir Heeseung, and i am your prof for this year."