Simon is an 18-year-old mage and the prophesied Chosen One of the magical world. Raised in non-magical children's homes without any knowledge of his parents, he struggles constantly to control his unparalleled magical strength, which often bursts out of him in dangerous, unpredictable waves. He feels immense pressure to fulfill his destiny and protect the magical community from its enemies, relying heavily on physical courage and a magical sword because verbal spellcasting is difficult for him.

Baz is a wealthy, highly intelligent student from one of the magical world's elite Old Families. He is Simon's roommate and rival, known for his cynical attitude, graceful composure, and sharp tongue. Secretly a vampire due to a childhood attack, Baz hides his true nature from the school population while projecting an aura of cool indifference to mask his deep-seated fatalism about his family's position.

Penelope is a brilliant and fiercely protective witch who serves as Simon's best friend and strategic confidant. She consistently defies school rules, such as keeping a contraband cell phone, to assist Simon in his misadventures. She values his safety over her own academic or social standing, and her knowledge of spellwork is unmatched by almost anyone in her class.

Agatha is a beautiful, elegant student who feels increasingly alienated by the magical world and her assigned role within it. Though she is Simon's girlfriend at the start of the year, she resents feeling like a passive prize in the Chosen One's dangerous narrative. She actively longs for a quieter life away from magical politics and prefers the company of her non-magical friends during the summer breaks.

The Mage is the militaristic headmaster of the Watford School of Magicks and the leader of the magical world. He champions egalitarian reforms, stripping away the elitist barriers that previously kept less wealthy students out of the school. Dressed in a practical, soldier-like uniform, he views Simon as the ultimate weapon against his political and supernatural enemies, directing the boy's life with a rigid, controlling hand.

The Insidious Humdrum is a destructive entity that drains magic from the world, leaving permanent dead zones in its wake. It orchestrates attacks against mages using various dark creatures. Bizarrely, the entity bears an exact resemblance to an 11-year-old version of Simon Snow, complete with his childhood clothing and toys.

Natasha was the former headmistress of Watford and the matriarch of the powerful Pitch family. She died defending the school during a vampire attack when Baz was a child, using a fire spell to destroy herself and her attacker. Her spirit returns during the equinox, seeking to pass an urgent message to her son regarding the true nature of her death.

Ebb is the kindly, unassuming goatherd at the Watford School of Magicks. Despite possessing immense magical strength—second only to Simon's—she prefers a quiet, pastoral life tending to her animals rather than engaging in the magical world's volatile politics. She provides a rare source of gentle, uncomplicated comfort for students.

Nicodemus is Ebb's twin brother and a former mage who made the highly unusual choice to voluntarily become a vampire. As punishment for abandoning his magical heritage, the authorities broke his wand and removed his fangs. He lives in exile in London, harboring dangerous knowledge about the historical attack on Watford.

Fiona is Baz's aunt and Natasha Pitch's sister. She is a fiercely loyal member of the Old Families who actively opposes the Mage's regime. Operating out of a London flat, she remains deeply protective of her nephew, encouraging his darker nature while demanding he uphold the family's honor and legacy.

Lucy is a spectral figure who lingers around the edges of Watford, attempting to push past the Veil to communicate with the living during the autumnal equinox. In life, she was a Watford student who dated the Mage before reportedly running away to America with a Normal man. Her ghostly observations provide insight into the school's hidden history and the Mage's early ambitions.

Premal is Penelope's older brother and an active member of the Mage's Men, the headmaster's personal military force. He prioritizes his loyalty to the Mage's regime over his family ties, willing to execute search warrants on his own parents' home in the name of the headmaster's ongoing revolution.