Aster is a 25-year-old medic living in the impoverished Q deck of the generation starship Matilda. Dark-skinned and queer, she moves through the ship's rigid social hierarchy using a special medical pass provided by the Surgeon General. She possesses a brilliant, highly literal mind, spending her rare free time maintaining a hidden botanarium and trying to decode her deceased mother's engineering journals.

Theo, also known as Surgeon, is a high-ranking doctor and member of Matilda's ruling class. Though he passes as white and holds immense privilege as the Lieutenant's nephew, he faces scrutiny for his lower-deck ancestry and non-conforming gender presentation. He acts as a mentor and protector to Aster, but constantly wrestles with the knowledge that his attention places her in physical danger.

Giselle is a resident of the lower decks who grew up alongside Aster. She bears severe psychological scars from the systematic violence inflicted by the ship's guards, often exhibiting dissociative behavior to cope. Her love for Aster is fierce but tangled with her trauma, making her actions erratic and unpredictable.

The Lieutenant is a high-ranking member of Matilda's upper deck elite, poised to take absolute control of the starship. He wields his unearned authority with casual cruelty, enabling and encouraging the brutal mistreatment of the lower-class residents. He views people from the lower decks as subhuman and openly despises his nephew's association with them.

Melusine is a pragmatic woman from the lower decks who works as a nanny for the children of the elite upper class. Within her own community, she functions as an improvised matriarch, coordinating resources and providing stability for the vulnerable. She leverages her connections to the upper decks to protect her chosen family.

Lune is Aster's deceased mother, who previously worked in the engineering section monitoring the ship's artificial sun, Baby. She recorded complex star charts and mechanical observations in coded notebooks, leaving behind a trail of clues regarding the ship's true trajectory and the cause of its mechanical failures.

Flick is a child living on a different lower deck than Aster. The starship's severe power outages plunge Flick's quarters into extreme cold, resulting in severe frostbite. Flick's medical emergency illustrates the dire living conditions forced upon the ship's underclass.

Sovereign Nicolaeus is the acting leader of the generation starship Matilda. He suffers from a strange, debilitating ailment causing partial blindness and confusion. His failing health creates a dangerous power vacuum that threatens to install an even more brutal regime over the lower decks.