Loner (2008)

Ha-Jung and Soon-na are best friends & students at the same school. However, while Soon-na has a happy and outgoing personality, Ha-Jung is often the target of extreme cruelty at the hands of bullies. After her best friend's strange suicide, the formerly bright and outgoing Soon-na locks herself in her room and becomes a recluse. She refuses to open the door to anyone and acts as if someone is in the room with her. Her personality degrades to the point where her uncle turns to his fiancee, a psychiatrist who specializes in hikikomori, for help. Hikikomori is a term to describe those who shut themselves off from society in reaction to trauma, usually by confining themselves to a room and avoiding human contact. The outcome is terrifying as supernatural occurrences collide with dark family secrets.