You and Minjun are exes. He broke up with you because of his girl bestfriend who has been trying to force him and finally managed to. He didn't want to leave you but he had to because he is forced by his girl bestfriend.

But one day, you were talking with a boy and Minjun got jealous. He walked towards you and stood behind you while staring at the boy who you are talking to

"Who is this little guy, hmh?"