The avatar of the GSV Sleeper Service - a tall, angular, black-dressed, cadaverous creature. Awkward and sometimes childlike in manner, Amorphia serves as the ship's physical representative during its interactions with Dajeil Gelian and later Genar-Hofoen. It reveals the ship's true nature as a secret SC weapon disguised as an Eccentric.

An ancient drone and long-time friend of the Seich family on Phage Rock, with personality elements dating back nine thousand years. It accompanies Ulver on her SC mission, serving as her chaperone and advisor. Practical and exasperated by Ulver's behaviour, it ultimately transfers to the Grey Area before returning to Phage.

A Culture citizen living in self-imposed isolation aboard the GSV Sleeper Service, nursing a decades-old grief. Her unresolved history with Genar-Hofoen becomes central to the events surrounding the Excession.

Colonel Alien-Befriender (first class) Fivetide Humidyear VII of the Winterhunter tribe, a boisterous Affronter Diplomatic Force officer and Genar-Hofoen's closest friend among the Affront. Loud, physically overwhelming and cheerfully aggressive, he epitomises the Affront's hearty but brutal culture.

A Culture citizen living as an ambassador among the Affront, a boisterous and deliberately cruel alien species. He has adapted to their ways with an enthusiasm that unsettles his Culture colleagues.

The solitary human caretaker of the secret Culture ship store at Pittance, a pathologically shy man who chose isolation as the only life he could tolerate. He builds exquisite scale models of sailing ships and tends small fires in the vast warship hangars. He is killed when Affronter forces breach the store, though his mind-state is saved by the Attitude Adjuster and later restored aboard the Sleeper Service.

A black bird aboard the Sleeper Service that serves as Dajeil's companion but is secretly a spy, reporting to an outside entity for forty years by encoding messages on bacteria deposited on departing Storees. Cranky, opinionated and self-important, it is eventually exposed and threatened with a scout missile. It later accompanies Ulver back to Phage Rock.

A Culture ship with a notorious reputation. Unlike other Minds, the Grey Area reads the thoughts of biological sentients without consent – an act considered deeply transgressive in Culture society. Shunned by other ships, it answers only to the name its peers use as an insult.

A sophisticated Elencher military drone from the Explorer Ship Peace Makes Plenty. After its ship is taken over by an unknown alien consciousness, it fights desperately to escape, transferring its mind-state to its twin before being captured. Its twin later destroys itself rather than be captured by the Affront, but its kamikaze attack conceals that it has already sent its mind-state elsewhere.

A General Systems Vehicle – one of the Culture's vast starships, home to billions. The Sleeper Service has been behaving eccentrically for decades, withdrawing from Culture society and filling its bays with tableaux of historical scenes. Its true motives are unknown.

A brilliant, beautiful and spoiled twenty-two-year-old from the ancient Phage Rock habitat, descended from one of its Founding Families. Recruited by SC to impersonate an old flame of Genar-Hofoen's on Tier, she is fierce, opinionated and quick-witted. Despite her privileged upbringing, she proves resourceful and surprisingly empathetic, helping to broker the reconciliation between Dajeil and Genar-Hofoen.

A winged AhForgetIt Tendency resident of Tier who spots an Elencher emergency message burned into the scar-hull of an Affronter light cruiser. He passes this crucial intelligence to the Tendency's vice-consul Lellius, inadvertently triggering the chain of events leading to the Excession's discovery.

The long-dead captain of the General Contact Ship Problem Child, who originally discovered the trillion-year-old sun and the enigmatic artifact in orbit around it over two thousand years ago. Her stored mind-state is used as a ruse to lure Genar-Hofoen to the Sleeper Service. She is later re-embodied and lives aboard the Sleeper Service with Dajeil and her daughter Ren.