Callum is a 12-year-old boy who walks with a painful limp due to a severe injury sustained as an infant. Raised by his father to fear and avoid magic, he actively tries to fail the entrance trial for the Magisterium. Despite his best efforts to suppress his abilities, he possesses a strong, unpredictable magical aptitude that secures him a place as an apprentice.

Jasper is an ambitious Iron Year student placed in Master Milagros's apprentice group. Coming from a family that lost everything in the Second War, he feels immense pressure to restore their wealth and reputation. He frequently targets Callum with rude comments, resenting him for taking a spot in Master Rufus's coveted group despite having terrible trial scores.

Aaron is a sporty, kind-hearted fellow apprentice in Master Rufus's group. Having grown up bouncing between orphanages and foster homes, he views the Magisterium as a miraculous opportunity for a better life. He keeps his belongings hyper-neat and possesses an inherent talent for magic.

Tamara is a highly ambitious Iron Year student who shares an apprenticeship group with Callum and Aaron. She comes from a disciplinarian magical family and feels intense pressure to exceed the achievements of her highly successful older sister. Initially high-strung and focused on the rules, she gradually learns to prioritize doing the right thing over following regulations.

Constantine Madden, widely feared as the Enemy of Death, is a dark and incredibly powerful mage. A former Magisterium student, he became corrupted in his quest for magical mastery and began performing dangerous experiments with chaos magic. His actions led to the horrific Cold Massacre and the creation of violent, Chaos-ridden creatures.

Alastair is Callum's father and a former mage who works as a debonair antique dealer. Having survived the traumatic Cold Massacre where his wife was murdered, he has completely sworn off magic and views the Magisterium as a place of monstrous danger. He is intensely protective of his son and desperate to keep him out of the magical world.

Master Rufus is a large, experienced mage and teacher at the Magisterium. He taught both of Callum's parents and uses highly unconventional, tedious teaching methods, such as making his apprentices sort individual grains of sand. He sees potential in students who struggle, choosing to train them as a form of personal penance for past mistakes.

Sarah is Callum's deceased mother and a former student of the Magisterium. She died during the Cold Massacre in an ice cave above La Rinconada when Callum was just an infant. Before her death, she managed to leave behind a cryptic and violent warning scratched into the ice.

Master Joseph is a former teacher at the Magisterium who instructed Constantine Madden. He supported his student's risky experiments involving chaos magic and chose to flee the school alongside him when the other Masters attempted to intervene.

Drew is a timid Iron Year student placed in Master Lemuel's group. He frequently struggles in class and suffers cruel mockery from his teacher, leading him to run away into the dangerous woods surrounding the Magisterium.

Havoc is a Chaos-ridden wolf puppy with swirling, kaleidoscopic eyes. Despite the dangerous reputation of chaos-infused animals, he shows an immediate, friendly affinity for Callum. The apprentices secretly adopt him and hide him within the Magisterium.

Master Milagros is a young female mage who teaches at the Magisterium. She selects Jasper for her apprentice group and leads outdoor practical lessons, such as teaching the students to balance on floating logs while juggling fireballs.

Master Lemuel is a strict and cruel teacher at the Magisterium. He bullies his apprentices, particularly Drew, by mocking their failures and denying them basic needs to pressure them into working harder.

Warren is a fire elemental in the shape of a lizard. He initially belongs to Master Rufus but strikes a deal with Callum, helping the boy travel through the Magisterium's dark passageways in exchange for his freedom.

Raven is Tamara's older sister who attended the Magisterium. She tragically died when she was permanently drawn into an element during her magical training, a loss that heavily influences Tamara's approach to magic.

Jericho is the twin brother of Constantine Madden. He acted as a magical counterweight for his brother's risky experiments and was tragically killed in the process.