Summary: A new HR teacher, Chun-woo is a nothing ordinary with his wily ambitions
and neglect of a bullied boy, Shin-woo. However, after unexpectedly
witnessing his great power, Shin-woo begs him to learn it. They could
start their lesson only after Shin-woo throws his body into deep river
to prove his will. However, Shin-woo did not know his master is stronger
than he can imagine. He was a killer with mastery of nine martial arts
who was hiding himself after a big mission!

Sources:
http://www.managreader.net
http://www.mangafox.com
http://www.onemanga.com
http://www.mangavolume.com
http://www.1000manga.com
http://www.mangastream.com