Sonya is a professionally trained assassin operating undercover as a regular high school student. Her background includes foreign assassin training, though specific origins or organizational details are undefined. Occupational hyper-alertness defines her, resulting in immediate, often violent defensive reactions to being startled or provoked, frequently targeting her classmate Yasuna Oribe during surprise interactions or pranks. Despite her cold, ruthless professional demeanor, she harbors fears of common elements like insects, ghosts, dogs, and other animals, contrasting her tough exterior.

Physically, she possesses long blonde hair styled in twintails secured by black ribbons, reaching her hips, and blueish-violet eyes. She wears the standard school uniform: a blazer, dress shirt, ankle socks, and tassel loafers, sometimes removing the blazer in warmer weather. A notable skill is her proficiency in weaponizing ordinary objects, including her own hair, showcasing improvisational combat frequently emphasized in the manga.

Her interactions are heavily defined by her relationship with Yasuna. Yasuna insists she is Sonya's best friend, a claim Sonya openly rejects, typically responding to Yasuna's antics with physical aggression. Despite this surface hostility, moments reveal underlying concern, such as collaborating to escape assassins or retrieve lost items. Their dynamic operates on a comedic "boke and tsukkomi" routine, with Yasuna's absurd behavior triggering Sonya's exasperated reactions. Agiri Goshiki, a ninja from the same organization, interacts with Sonya occasionally, though their relationship remains professionally ambiguous and minimally developed.

Across all media, including the anime series and OVA, her core traits remain consistent. Situational episodes—involving beach outings, cultural festivals, or snow activities—highlight reactions rather than significant evolution. For example, during a snowy incident, she destroys numerous snowmen while pragmatically searching for a lost wallet. The finale briefly acknowledges her bond with Yasuna when Yasuna expresses concern about Sonya's dangerous lifestyle, hinting at mutual care beneath their antagonism. The OVA continues established patterns without new development. Her character design intentionally parodies Nagi Sanzenin's hairstyle from another series, an aesthetic choice without narrative significance.