- Complete Monster: Believing himself to have been born without the ability to be happy, apartment concierge César Manso only finds satisfaction in making everyone around him unhappy. While he easily upsets and turns most of his tenants against each other, Clara proves to be challenging, so he decides to ruin her life for no reason. Every night, he hides under her bed, chloroforms her then rapes her, before injecting irritants in her cosmetics and putting cockroach eggs all over her place. During the day, he harasses her with anonymous letters, and later frames the cleaning lady's son for this. When Clara's boyfriend finds out, César kills him with a mirror shard and stages his death as a suicide. He also sadistically torments his dying mother by telling her about his deeds and threatens a little girl who knows about his murder of Clara's boyfriend and tries blackmail him. Finding happiness upon learning that Clara is pregnant with his baby, he sends her a letter taking pride in ruining her existence.
- Moral Event Horizon: César, while doing bad things for great part of the movie, crosses it when he kills Clara's boyfriend.
- Paranoia Fuel: Do you trust the concierge with your keys? You won't after watching this movie.
- What an Idiot!:
- When Clara and her boyfriend find a serial killers starter pack under Claras bed, they quickly lose interest in it and switch their attention to other things.
- Claras boyfriend conveniently confronts César face-to-face and in a location with no witnesses instead of informing Clara and the police or at least police alone, in case he didnt want to involve Clara for her sake.
- The little girl similarly doesnt inform the investigators and her parents when she witnesses the commotion after Claras boyfriends death, and decides to further provoke a potential killer with a silly letter, for some reason.
- The Woobie:
- Clara. She's stalked and unconsciously raped by César every night. After her boyfriend apparently committed suicide (he's been killed by César actually) she's left alone with a baby who is revealed to be the result of César's violation. It's also implied that his rapist will never be caught.
- Véronica. She's a sweet, timid old woman who comes off as insecure, and she's never anything short of nice to everybody, even César, unaware of what a heartless monster he really is. The last we ever see of her, her spirit has, in all likelihood, been crushed when all she ever tried to do was comfort César after he'd just been fired from his job.
YMMV / Sleep Tight