Ji-na Yeo

Moving into "Il Mare", his new apartment building, Sung-hyun finds a delivery in his mailbox, a love letter sent by a woman called Eun-ju to the man who has left her (imdb)

Five months after his release from prison after serving a twelve-year sentence, arsonist Yeo Hee-soo terrorizes the city of Seoul with a series of deadly blazes. Yeo rigs each fire so that a second, far more lethal conflagration ignites shortly after the firefighters have arrived, causing further casualties. He then turns his attention to those members of the department he feels are interfering with his "mission", which develops into a game of cat and mouse with veteran fireman Jo Sang-woo. (en.wikipedia.org)

An expelled boxing champion, who now is a high-school teacher, witnesses intolerable violence and throws her first punch to build justice against it, while putting on a mask.