Spensa is a determined teenager living in the Igneous cavern on the planet Detritus. Scavenging rats and mushrooms to support her marginalized family, she possesses a fierce ambition to join the Defiant Defense Force (DDF) as a pilot. She models her behavior on the bravado of ancient Earth heroes to mask her insecurities about her family's ruined reputation. Despite the societal stigma against her, she remains fiercely loyal to her loved ones and her culture's ideals.

Captain Cobb is a gruff, scarred DDF instructor tasked with training the newest batch of cadets. As Zeen Nightshade's former wingmate, he carries deep personal trauma from the Battle of Alta but is one of the few authority figures willing to give Spensa a fair chance. He prioritizes survival skills over aggressive combat tactics.

Judy Evans is the formidable admiral of the Defiant Defense Force. She leads the desperate war effort against the Krell and is singularly determined to prevent Spensa from becoming a pilot. Driven by a deep fear of repeating past disasters, she uses her bureaucratic and military power to construct insurmountable obstacles for Spensa.

Zeen is Spensa's late father and a former pilot for the DDF. Known for his actions at the Battle of Alta, he is officially remembered by the military and Defiant society as a coward who fled from the enemy. His ruined reputation dictates the poor living conditions of his family, though his daughter firmly believes in his underlying heroism.

Gran-Gran is Spensa's paternal grandmother, serving as the family's link to humanity's distant past. She maintains the tradition of telling ancient stories from Earth, such as the tale of Beowulf, which heavily shapes Spensa's worldview. She possesses an independent perspective on the skills of early space navigators and offers Spensa unconditional support.

Spensa's surviving mother struggles to provide for her family in the Igneous cavern after her husband's disgrace. Refusing to accept the government's official story about Zeen's cowardice in exchange for a comfortable job, she lives a marginalized life. She harbors a deep distrust of the DDF and worries constantly about Spensa's ambitions.

Rodge is Spensa's loyal childhood best friend who initially joins her in testing for the DDF flight academy. Unlike Spensa, he is anxious about flying and harbors a much stronger aptitude for engineering and mechanical design. He provides crucial technical support when Spensa makes an unusual discovery in the caverns.

Jorgen is the wealthy, privileged son of a First Citizen and a National Assembly Leader. Appointed as the flightleader of Spensa's cadet group due to his extensive prior simulator experience, he expects immediate respect. He feels trapped by his family's political expectations and struggles to prove himself as a true leader.

Hudiya is a fellow cadet in Skyward Flight who hails from the same cavern city as Spensa. Initially wary of associating with the daughter of a known coward, she is highly competitive and determined to project bravery at all times. She makes a fierce vow never to show fear, driving her to take significant risks in the cockpit.

M-Bot is a highly advanced, mysterious ship equipped with artificial intelligence that Spensa discovers crashed in a cavern. Having lain dormant for over a century, M-Bot possesses a quirky personality, an obsession with cataloging mushrooms, and vast gaps in his memory banks regarding his origins or his original pilot.

Kimmalyn is a genuinely open and friendly cadet in Skyward Flight. She immediately treats Spensa with warmth and acceptance despite Spensa's tarnished reputation. She brings a much-needed sense of kindness and camaraderie to the highly stressful flight academy environment, and surprises her peers with her sharp shooting skills.

FM is a sharp-minded cadet in Skyward Flight who harbors anti-authoritarian views. Despite being willing to fight the Krell to protect her people, she openly criticizes the DDF's autocratic military government and the strict societal class divisions on Detritus. She becomes a steady source of support for Spensa.

Arturo is a wealthy, upper-class cadet in Skyward Flight. Like Jorgen, his family expects him to serve only briefly in the DDF before transitioning into a safer, prestigious political career, a pressure that dictates much of his trajectory in the academy. He asks pointed questions about the Krell that challenge DDF secrecy.

Nedd is a cadet in Skyward Flight who purposefully projects a less intelligent, easygoing persona than he actually possesses. He treats Spensa fairly from the beginning and carries the heavy burden of having older brothers who are active full pilots in the DDF, exposing him constantly to the realities of the war.

Morningtide is a quiet cadet in Skyward Flight who speaks very little English. Her taciturn nature is initially mistaken for hostility by Spensa until the language barrier is revealed. She participates earnestly in the rigorous training required of all cadets.

Bim is an eager and enthusiastic cadet in Skyward Flight. He is highly passionate about combat and eager to engage the enemy, demonstrating a trigger-happy nature that sometimes overrides his instructor's cautious lessons.

Doomslug is a bright yellow and blue slug-like creature that Spensa discovers in the cavern alongside the abandoned ship. It frequently watches her work, making distinct trilling noises that sound almost like spoken words, serving as a strange but comforting companion in her isolation.

Mrs. Vmeer is Spensa's teacher in Igneous cavern who tries to steer her away from taking the pilot's exam. Knowing the intense prejudice held by DDF leadership, she attempts to convince Spensa to accept a mundane job, warning her that she will never be allowed to pass the test.

Dia is a classmate of Spensa's in Igneous cavern who openly mocks her ambitions. As the daughter of Mrs. Vmeer, she overhears and eagerly shares the rumor that Admiral Ironsides has rigged the flight school tests to ensure Spensa fails.