Tomoshibi (2004)

Yuko Yajima works at a library. To her co-workers she appears as an extremely shy and introverted person who hardly lets anyone know her true feelings and never smiles. However her interest in a man living in room 202, Mamoru Kosino, leads Yuko into some risque behavior. The interest turns into obsession, as Yuko's jealousy turns into a thrilling game where she sneaks into apartments and scares tenants away, hoping to have Mamoru all to herself.