Bok-Soon (Kim Go-Eun) lives with her younger sibling. She runs a street stand. Bok-Soon is slow, but when people angers her she goes crazy. That’s why she is known as the “crazy woman”.
One day, cold-blooded killer Tae-Soo (Lee Min-Ki) appears in front of her. To hide his secret, he kills Bok-Soon’s younger sibling. Bok-Soon then goes after Tae-Soo for revenge, but Tae-Soo goes after Bok-Soon.

http://asianwiki.com/Monster_-_Korean_Movie

Kim Go Eun on Bok-soon:

“I thought Bok-soon was sort of, I don’t know, cute when I first read the script,” Kim said, when asked what drew her to “Monster” and her character. “She was like some cartoon character. She has this developmental disability, but she’s simple and kind, loves her sister and is almost uncannily responsible. She’s spent her entire life selling vegetables in a market, so she’s very tough, and determined too. I thought it was interesting how such a character becomes this monster as she goes through a series of unfortunate events. I also liked the idea of a female character fighting against a serial killer.”

Eun-gyo returns as a ‘Monster’ by Claire Lee

Directed by: Hwang In-ho
Written by: Hwang In-ho
Starring: Kim Go-eun, Lee Min-ki
Release date: March 13, 2014

Photo source: hancinema.net

Additional sources: