Stalker: Persona (2019)

Mi-jeong, Ji-hye and Ji-hye's boyfriend, Geon-soo, live in the same house. They enjoy having sex without being disturbed by each other. While Ji-hye and Geon-soo are out, Mi-jeong finds Geon-soo's camera and finds out that he is the one who has been stalking her. Mi-jeong wants to ask him about it, but she's already fallen for him. In the end, they turn into a secret relationship. One day, Mi-jeong suggests they stalk Ji-hye...