Luke's older brother and a brilliant but emotionally detached scientist whose pursuit of knowledge is characterized by ambition and a lack of ethical consideration, serving as a foil to Luke.

A competent Navy officer and submersible pilot whose sense of duty and resilience guide her actions, but she faces her own psychological challenges while navigating the deep-sea mission.

A manipulative and sadistic figure from Luke's past, appearing only in flashbacks and dreams, whose psychological abuse profoundly affects Luke's psyche.

A scientist at the Trieste station whose journal chronicles a descent into madness, illustrating the psychological impact of the station's environment.

A chaos theorist whose intellect falters under the Trieste's unpredictable forces, leading him to paranoia and ultimately a tragic demise.

Clayton's partner on the Hesperus station, who embodies scientific ambition and provides background on the discovery of a substance called ambrosia.

The skipper who transports Luke, whose personal story of loss due to a global pandemic highlights the broader human stakes of the mission.