Wealthy, handsome, and a collector of beautiful things, Desi has been obsessed with Amy for most of his life. They dated in boarding school, where Desi was possessive and territorial with Amy. Amy later spreads rumors that Desi stalked her post-breakup, and that he tried to kill himself in her dorm room. Despite this betrayal, Desi remains infatuated with Amy, harboring feelings for her years after they stopped seeing each other. He writes her regular letters, which Amy pretends to throw away in front of Nick but secretly responds to. Desi is close with his mother, a woman with a strange, erotic energy. She hates Amy, but as Nick observes, she also bears a striking resemblance to her. Desi, it seems, has a very particular type. He's desperate for Amy's affection and approval, willing to go to great lengths to accommodate her and to accept any humiliation in order to have her. When he dates other women, they are always people who are struggling with substance abuse issues or depression. Being with vulnerable women appeals to him; Amy says that he is at his best when he is sitting by a bedside. He thinks of himself as a white knight and enjoys swooping in to save the day.
He becomes an important figure in Amy's plan when she decides to return to her old life. Desi can't resist her wiles when she reaches out to him, and after she meets him and spins a tale of a daring escape from Nick, he’s hooked on her all over again. He immediately agrees to help her, whisking her away to his secluded lakeside cabin. Once they get there, however, it's clear that this “help” is really just the fulfilment of Desi’s long-cherished fantasy. Amy starts to wonder if she's seeing the bars of a gilded cage pretty quickly after she gets there, and it's soon clear that she is.
Desi is not actually interested in Amy as a person: he just wants her to be the same perfect, pristine object that he’s worshipped since boarding school. He is desperate to collect and keep her for himself, and he immediately begins to take control of every aspect of her day-to-day life, controlling her food, her hair, her clothes, and her ability to enter and exit his house. He wants her to look and dress “like herself,” and won’t be satisfied with less. The chalet, which was supposed to be a refuge for Amy, quickly becomes her prison. She recognizes Desi’s overbearing care and control as a watered-down version of her own obsessive schemes and plans. She ends up killing Desi and framing him to escape. This task is made far easier by the fact that Desi sincerely believes she is in love with him. He’s so self-absorbed that he can’t see Amy’s manipulations for what they are, even after everything she’s done to deceive and damage him.