The Lethe is Atlas's unregistered, utilitarian torchShip, used to hide his presence in the Rim and hold Lysander after the Kalyke ambush.
The Lethe is Atlas's well-used covert torchShip, deliberately kept off the books while the more conspicuous Styx is believed to be carrying the Fear Knight elsewhere. It brought Atlas to the Rim and now lies on Io, its windowless cabins, muted corridors, medical bay, and common rooms housing the Gorgon force responsible for Kalyke.
Lysander is held aboard it for ten days after capture. From the ship he hears the volcanoes of Io, meets Rhone after the Garter breach, and is brought to Atlas's medical bay for the revelation of the long plan. The Lethe is not presented as a glamorous flagship; its anonymity and practical maintenance are what make it useful to a commander who needs absence to be as believable as presence.
With the Garter under attack and Sungrave's defense codes extracted, the Lethe is the mobile nerve center from which Atlas turns the Kalyke betrayal into the next phase of the invasion.
A utilitarian, worn black-operations torchShip with gloomy corridors, small windowless cabins, a medical bay, common rooms, and no public record tying it to its master.
The Lethe gives Atlas a hidden command post and prison from which he can execute the Rim operation without exposing himself to ordinary Society scrutiny.