This is a great movie that I recommend to watch.

Ji-ho is a promising pianist who has been studying and playing abroad for years, but her career takes a blow when she is struck with stage fright. Returning home, Ji-ho is reunited with her old school friend, Seung-ryong, who, although now in his twenties, has been left with the mind of a six year old following an accident.
Seung-ryong's only family is his younger sister, Ji-in, and he takes care of her by trying to sell toast outside her school, much to her embarrassment. Ji-in later becomes ill, and Seung-ryong's other friend, Sang-soo, falls into trouble with some gangsters. With all of his friends and family facing problems, Seung-ryong becomes an unlikely saviour.