Sonya balances life as a high school student with her secret career as a trained assassin. She maintains her cover by attending classes, her blonde hair usually in pigtails and blue eyes framed by a standard schoolgirl uniform of blazer, dress shirt, skirt, and loafers. Her assassin's existence demands constant vigilance, causing her to react defensively—often with violence—when startled, especially by classmate Yasuna Oribe's pranks or sudden approaches. Despite this aggressive response to perceived threats, Sonya harbors fears of common phobias like ghosts, cockroaches, wild animals, and dogs.

Specialized assassin training forms her background, though specifics about her origins or mentors remain undefined. She maintains professional precision in her work but struggles to reconcile her covert identity with ordinary school life, fueling heightened paranoia. Her relationship with Yasuna is complex: while Yasuna considers them best friends and constantly seeks her out, Sonya frequently responds to Yasuna's antics with physical retaliation, driven by irritation or reflexive self-defense. Yet, beneath this friction, Sonya consistently demonstrates underlying loyalty. She repeatedly intervenes to prevent Yasuna's impulsive behavior from leading to danger and remains a constant presence in her life. This dynamic casts Sonya as both a reluctant guardian and an exasperated companion who, despite her violent reactions, holds no lasting resentment toward Yasuna.

Sonya's skills include proficiency in close-quarters combat, stealth tactics, and rapid threat assessment, all essential to her assassination work. She generally exhibits pragmatism and level-headedness compared to peers, though disruptions quickly trigger her short temper. Her core traits—ruthlessness in her profession, loyalty amid conflict, and susceptibility to fear—persist as defining characteristics.