Less than Zero has an extensive cast of characters; the ones listed play major roles throughout the novel.
- Clay
- The 18-year-old protagonist, a student at Camden College in New Hampshire, who comes home to Los Angeles for Christmas and meets his old friends. He revives his old life: parties, concerts, drugs, sex, the city. Clay has brief affairs (a male USC student named Griffin (pp. 35–39), and an unnamed female character (pp. 120–122)), but he goes through periods of apathy and longing for his girlfriend Blair.
- Blair
- Clay's ex-girlfriend who is studying at USC. Clay is unsure how he feels about her; neither has he been faithful to her. They vacation together and though at first they enjoy the experience, it eventually becomes tedious and ends on a sour note.
- Julian Wells
- Clay's friend from grade school and high school. Julian is often described as "thin" and many of Clay's friends declare him "completely fucked up". Julian has become a heroin addict and a prostitute.
- Trent Burroughs
- Another of Clay's friends, a model who attends UCLA. He says things that Clay doesn't understand and Clay gradually becomes disenchanted with him. Trent is shown as increasingly unethical and immoral, raping a drugged twelve-year-old girl in his penultimate scene.
- Rip Millar
- Clay's dealer. Sporting a fedora and a penthouse on Wilshire Boulevard, Rip is also a DJ, but feels his trust fund "might never run out." At the end of the story, he shows Clay and other boys a 12-year-old girl naked, drugged and tied to his bed to be a sex slave. When a distressed Clay questions Rip as to why he has done this, Rip replies "Why not? What the hell?"
- Daniel Carter
- Daniel is another student who attends Camden and is from Los Angeles. Many of the characters think he's gay. In his earlier appearances, he was worried he had gotten a girl from Camden, Vanden, pregnant, but then doesn't seem to care. In his final appearance, he tells Clay he won't return to Camden, opting instead to stay in LA and write a screenplay. (Vanden, who isn't seen in the book, later appears in American Psycho as well as The Rules of Attraction, in which she and Clay are briefly "involved".)
- The Handsome Dunce
- A minor character who has a cameo speaking to Clay at Blair's Christmas party. He goes on to appear in The Rules of Attraction, where it is revealed that he goes to Camden College and that his name is Steve.
- Kim
- One of Blair's friends. During the course of the book she's never sure where her film producer mother is and only knows what she reads in trade papers. At one point in the novel, she and Clay both agitate each other notably when they repeatedly question each other, "What do you do?", "What do you do?", this ending with Kim finally replying, "Don't ask me because...I don't know."
- Alana
- Another one of Blair's friends. She has an abortion and comes to Clay afterwards. Clay lets her stay in his room for the night while he lies by the pool until dawn. When he goes back up, she informs him she has bled a lot and feels weak before thanking him. When he asks "what for?" she says "I don't know" and leaves. When Clay flushes his toilet, it becomes clogged up with tissue and blood clouds the water. Clay puts the lid down as "there's nothing else for me to do".
- Muriel
- An anorexic girl who Clay visits at a rehabilitation center. She appears a few times in the novel, most notably at a New Year's party where she shoots heroin publicly while laughing and crying.
- Finn Delaney
- Julian's pimp, who "helps" him pay off his drug debt. In public, he acts kindly towards Julian, calling him "Julie" and "his best boy" and often showing him off to his clients, but in private he abuses Julian emotionally and sexually and forces him to shoot up heroin.
- Clay's family
- Clay has two sisters, aged 13 and 15, although his narration suggests he cannot distinguish between them and is unsure of their age. His parents are separated; his mother occupies their house while his father lives in an apartment. His mom has no job but nonetheless lives a life of luxury off of her ex-husband's large alimony payments, and his dad is "in the film business", with an office in Century City. In flashbacks, Clay talks about his grandfather, proprietor of several hotels, and his grandmother, now deceased.
