It is ultimately revealed that Su-mi and her father were alone in the house the entire time. Su-yeon and Eun-joo were merely hallucinatory manifestations of Su-mi's dissociative identity disorder. Throughout the film, Su-mi simultaneously switched personalities, acting as herself and Eun-joo. She hallucinated Su-yeon as a result of not being able to accept her death. In her "Eun-joo" personality, Su-mi tries to sleep with her father as his "wife" and pretends to abuse Su-yeon by injuring herself and putting a porcelain doll in a sack and whipping it. It's also revealed that Su-mi was the one responsible for killing the pet bird.