Cordelia (2020) on Netflix

Following an incident in her past, Cordelia now lives with her twin sister, Caroline in their late father's basement flat.

Cordelia is a troubled, damaged soul who is only just recovering after some unnamed trauma; she is an actor rehearsing a play and comes to stay in a creepy London mansion flat occupied by her twin sister Caroline and Caroline's boyfriend Matt. When they leave her alone there Cordelia strikes up a friendship with Frank the charming, but strange and unreliable young man they can hear practicing his cello in the upstairs flat - a relationship which quickly becomes very disturbing.

—Canon y mus

Cordelia (Campbell-Hughes) is a troubled young woman living with the ongoing effects of trauma. She comes to stay in a London mansion flat occupied by her twin sister and her boyfriend. When they go out of town, Cordelia strikes up a friendship with Frank (Flynn), the charming but strange young man who lives upstairs - a relationship which quickly becomes very disturbing.