Clark is a struggling furniture-store owner and a failed architect. He is intelligent, creative, stubborn, emotionally troubled, and increasingly obsessed with the Backrooms. His business is failing, his marriage has fallen apart, and he struggles with alcoholism. Despite his problems, he can be charismatic and convincing when he wants someone to believe him. He becomes fascinated by the strange doorway beneath his store and begins spending more and more time investigating it. He originally hired Kat as an employee and assistant manager, and Bobby works at the store as well. Clark relies on both of them to help with the business and eventually convinces them to enter the Backrooms with him. He initially wants to understand the Backrooms, but his obsession gradually becomes more important to him than his normal life.

Dr. Mary Kline is Clark's therapist. She is intelligent, observant, skeptical, compassionate, and emotionally guarded. Mary has her own difficult history: her childhood home was demolished, and she grew up dealing with the trauma surrounding her mother's problems and institutionalization. She is accustomed to helping troubled people while keeping her own emotions under control. Clark becomes one of her patients and tells her about the strange Backrooms, but she initially believes his obsession is connected to his personal problems rather than a real alternate dimension. After Clark disappears and leaves a disturbing message, Mary becomes concerned enough to investigate his furniture store herself. Her relationship with Clark is complicated because she genuinely wants to help him but refuses to simply agree with his increasingly bizarre beliefs.

Kat Taylor is Clark's employee and assistant manager at his struggling furniture store. She is practical, confident, loyal, and generally more grounded than Clark. She knows the business is struggling but continues working there. Because she works closely with Clark, she becomes involved when he starts investigating the strange phenomenon beneath the store. Clark eventually convinces Kat and Bobby to enter the Backrooms with him so they can record evidence of what he has discovered. Kat is skeptical of Clark's obsession at first, but she is willing to help him and becomes trapped in the Backrooms after Bobby is attacked and she becomes separated from Clark.

Robert "Bobby" Franklin is one of Clark's employees. He is relatively ordinary compared with Clark and the people connected to Async. Bobby is friendly, cooperative, and willing to help Kat, but he is not nearly as obsessed with the Backrooms as Clark is. Clark recruits Bobby and Kat to help film a television commercial for the furniture store and later persuades them to accompany him into the Backrooms to document his discovery. Bobby enters the strange dimension mainly because he trusts Kat and is willing to help Clark. His involvement becomes much more serious when he is separated from the others and attacked by the entity inside the Backrooms.

Captain Clark, commonly known as Pirate Clark, is a gigantic and distorted entity inside the Backrooms that resembles Clark while wearing the pirate costume used by Clark's furniture store. It is silent, unpredictable, frightening, and extremely dangerous. The creature appears connected to Clark's identity, memories, and obsession with the Backrooms. Clark eventually becomes convinced that he has accepted this creature as part of himself rather than treating it as an enemy. Pirate Clark does not speak or explain itself, making its true nature unknown. It eventually turns against Clark and then pursues Mary through the Backrooms.