Aster Grey
Aster Grey is the central character, and she appears throughout the story. Aster is 25 years old, and her contribution to helping to save the life of Flick is one of her greatest achievements throughout the book. Aster lives among the people in the lower castle, and they are all affected by power outages that have led to poor living standards. For instance, the lower deck residents suffer from extreme cold weather, which leads to frostbite that affects Flick's leg.
Aster uses the skills to surgically remove the gangrenous foot from Flick and save his life. Aster has a good relationship with her sister Giselle who is fond of reading her late mother's journals. Aster is depicted as an intelligent woman who learns new concepts very first. For instance, Aster is approached by Theo Smith to help him formulate a cure for a strange disease.
Flick
Flick is the child living on the lower castle, which is highly affected by frostbites due to very low temperatures. Flick is helped by Aster, who uses her skills to remove the gangrenous foot from Flick surgically. Despite Flick being a young child, he appreciates Aster's help and hopes the living conditions in the lower deck will improve.
Surgeon General Theo Smith
Theo Smith is a surgeon by profession. When Smith fails to get the cure for the mysterious disease affecting the ship's acting leader, he requests Aster to help him formulate a cure. Aster is reluctant to join Smith in his noble profession because she is still exploring her talents and opportunities to see where she fits. However, she agrees to help Smith find the cure but requests more time to do her research.
Giselle
Giselle is Aster's sister, and she is friendly. Giselle is fond of reading, and she steals her dead mother's journals to read. However, the journals are written using the coding language, making it difficult for her to follow at first. Later, Giselle starts to understand the coding, and she translates most of the writings to Aster. The journals cover various diseases and their respective symptoms.
Lune
Lune is the late mother of Aster and Giselle. When she was alive, she loved writing engineering journals and notebooks and documented her sickness and symptoms. During her lifetime, she influenced Giselle and Aster to love reading and to do research on various topics.
Sovereign Nicolaeus
Nicolaeus is the ship’s acting leader suffering from a mysterious disease threatening to kill him. Smith is struggling to help Nicolaeus but has failed to get the best cure for his sickness. Nicolaus's symptoms include fractional blindness, perplexity, and dissociative behavior. Smith requests Aster to help him find the appropriate cure to save the life of Nicolaeus, to which she reluctantly agrees. Unfortunately, Nicolaeus dies before the actual cure is discovered. After conducting the autopsy of Nicolaeus's body, Aster discovers that he was killed by metal poisoning, which also killed her mother. Later, Smith plays a vital role in helping Aster explore the ship's engineering section, where her mother once worked before she died.
Lieutenant Theo's Uncle
The Lieutenant is Theo's uncle, and he takes the position of Nicolaeus after he dies. According to Aster, the Lieutenant is cruel and does not help anybody who needs help. Unfortunately, the Lieutenant is the new acting leader of the ship, and people must strictly follow his directions.
