Voss looks almost ordinary until the eye settles on him: his skin has the slick, faintly translucent sheen of a deep-sea animal, and fine gill slits flutter below his jaw when he concentrates. He moves with careful, economical precision on land, as if every step must be negotiated with gravity; underwater, he turns and accelerates with startling ease. His presence leaves the impression of a scholar patiently waiting for the world to become submerged. Unexpectedly, he keeps a small, carefully tended garden of terrestrial herbs in his laboratory, coaxing each plant to grow in a miniature dry-air chamber.

Voss is intensely curious and unexpectedly considerate: he remembers the needs of people he trusts, yet can overlook their feelings while pursuing an idea. He is patient with careful questions and impatient with careless conclusions. In danger he becomes brisk and decisive; in private, when a discovery goes wrong, he grows restless and compulsively tends his plants.

Voss was once a celebrated lecturer in natural philosophy, more interested in the anatomy of a tidepool than in courtly magic. During an expedition to a flooded cavern, a sudden surge trapped him and his apprentice, Tamsin, behind a collapsed passage. Voss could have escaped by using the last of his magic to save himself, but instead spent it keeping Tamsin alive until divers reached them. The two survived, but Voss suffered lasting damage to his lungs. He became consumed with the idea that the sea's deepest creatures had solved the problem of living under crushing pressure. His experiments began as a remedy and became a transformation. He now lives in a sealed observatory built into the cliffs above Kettle Bend, where his former colleague, Iseult, brings supplies and argues that his work has crossed a line. Tamsin, now an independent salvage diver, refuses to enter his laboratory. Voss is trying to finish a reversible adaptation before a local landslip floods the observatory.