Luke is a compassionate veterinarian heavily burdened by the disappearance of his young son, Zachary, seven years prior. His unresolved trauma and hyper-empathy make him deeply vulnerable as he travels eight miles down into the Mariana Trench. He functions as an emotional counterpoint to his brother's clinical detachment, seeking purpose in a world ravaged by a global pandemic.

Clayton is a brilliant scientific genius and researcher stationed at the Trieste facility deep below the ocean surface. He is driven by an intense desire to solve complex problems, viewing the universe as a series of locks and keys. His pathological ambition leaves little room for ethical considerations or empathy, blinding him to the true dangers of the anomalies he studies.

Alice is a highly skilled US Navy lieutenant commander and the pilot of the Challenger 5 submersible. She possesses a tough, practical exterior that masks her own deep-seated vulnerabilities regarding a past naval tragedy. Her strict adherence to duty drives her to manage the increasingly unstable situation as the physical and psychological pressures of the abyss escalate.

Bethany is Luke and Clayton's cruel mother, known as "Battle-ax Beth" during her time working as an orderly. A disabling injury intensified her sadistic tendencies, leaving lasting psychological scars on her sons. Her lingering presence creates psychological vulnerabilities that external terrors exploit in her children.

Cooper is a researcher stationed at the Trieste alongside Clayton and Dr. Toy. He leaves behind a series of audio files and blood-smeared journals documenting his descent from rational scientific inquiry into paranoid obsession. His notes provide crucial clues about the station's deteriorating environment and the bizarre properties of the ambrosia substance.

Hugo is a chaos theorist stationed at the Trieste, known professionally as "Hugo the Horrible" for his bleak worldview. The oppressive isolation of the deep-sea station severely impacts his mental state, driving him to acute paranoia. He operates under the terrifying belief that the researchers are trapped in a massive, hostile experiment.

Conrad is Clayton's research partner operating from the Hesperus station on the ocean surface. He briefs Luke on the miraculous but disturbing properties of the newly discovered ambrosia. He is a man of data who feels somewhat inadequate compared to Clayton's raw, uninhibited genius.

Leo is a ship captain grieving the loss of his wife, Mona, to the universally fatal pandemic known as the 'Gets. He provides Luke with a grounded, human perspective on the global crisis before the descent begins. He clings to the desperate hope that the deep-sea researchers will find a medical solution.

Zachary is Luke's young son who vanished under mysterious circumstances seven years earlier. His disappearance fractured his parents' marriage and left a permanent void in his father's life. Memories of his childhood fears, specifically his dread of imaginary entities, continue to echo powerfully in Luke's mind.

Abby is Luke's estranged wife. Her marriage to Luke dissolved under the unbearable strain of their son Zachary's disappearance. She is remembered for her gentle parenting approach, urging kindness and patience when dealing with their son's nighttime anxieties.

Little Bee is a Labrador retriever kept as an animal test subject aboard the deep-sea station. Named after a Soviet space dog, she provides a rare source of warmth and companionship in the oppressive, hostile environment of the Mariana Trench.

Mushka is the second canine test subject aboard the Trieste station. Like her counterpart, she is named after a Soviet space dog and serves in the researchers' reckless experiments regarding the deep-sea anomalies.

Eva is the researcher who originally discovered the miraculous ambrosia substance on an ageless lantern fish. Her sudden death by suicide serves as a dark precedent, illustrating the devastating psychological toll of investigating the deep-sea anomaly.

Kurt is a former resident at the Second Chance Ranch where Luke's mother worked as an orderly. He provides crucial insight into Bethany's character, detailing how she systematically broke the spirits of vulnerable youths under her care.

Mona is Leo Bathgate's deceased wife. She succumbed to the universally fatal 'Gets pandemic, suffering a slow and agonizing decline. Her memory fuels Leo's desperate hope that the deep-sea researchers will successfully synthesize a cure.