Nathanael is a sensitive, highly imaginative university student whose life is overshadowed by a severe childhood trauma. He favors a romantic, emotionally driven view of the world, often clashing with those who prefer logic. He struggles to distinguish his inner fears from reality, particularly regarding a childhood boogeyman he believes is currently stalking him.

Clara is Nathanael's fiancée and Lothar's sister. Rooted firmly in Enlightenment rationality, she possesses a calm, cheerful, and practical demeanor. She attempts to ground Nathanael's wild fears using logic and reason, diagnosing his terrors as psychological projections rather than physical threats.

Coppelius is an old, grotesque German advocate who frequently visited Nathanael's family during his childhood. He conducted mysterious, nocturnal alchemical experiments with Nathanael's father. Nathanael closely associates this frightening, abrasive man with the mythical Sandman who steals children's eyes.

Lothar is Clara's brother and Nathanael's close friend. He serves as a steadying presence in their social circle, sharing some of his sister's rational grounding while maintaining a protective, brotherly bond with Nathanael.

Coppola is an Italian barometer and optical glass salesman who operates near the university. His arrival deeply distresses Nathanael, who finds the salesman's appearance uncomfortably similar to the dreaded Coppelius from his childhood.

Olimpia is introduced as the beautiful daughter of the university professor Spalanzani. She maintains a notably rigid, silent, and isolated existence within her father's home, drawing Nathanael's intense romantic fixation from afar as he projects his desires onto her quiet demeanor.

Spalanzani is a physics professor at Nathanael's university. He keeps a notoriously private household and closely guards his daughter, Olimpia, from the outside world, controlling who is allowed to interact with her.

Nathanael's father was a loving parent whose secret, late-night alchemical studies ultimately led to his death in an explosion. His mysterious association with the fearsome advocate Coppelius casts a long shadow over his son's adult life.

Nathanael's mother is a protective figure from his youth who tried to shield her family from the unpleasantness of her husband's nocturnal associate. She first introduces Nathanael to a mild, harmless version of the Sandman myth.

The governess who helps raise Clara supplies the young, impressionable Nathanael with a gruesome, horrifying folk tale about the Sandman, permanently scarring his psyche.