Sonya Kiryū is a first-year student at St. Francesca Academy with a petite, slim build, standing 145 cm tall and weighing 34 kg. She has pale skin, bright blond hair that falls to her waist, and blue or cyan eyes. Her typical attire includes a school uniform, a cross necklace, and ribbon accessories, though she is also seen in a maid's dress or a veil, reflecting her religious life and part-time work.

She carries herself with a docile, serious, and refined manner, speaking politely and giving off an angelic, mysterious atmosphere. Kind and gentle by nature, she is also deeply shy and introverted, rarely making eye contact or speaking about her personal history. A good listener who does not complain, her reserved behavior has often made forming friendships difficult. She serves as a novice nun and dedicates much of her time to cleaning the local church.

Sonya is of mixed Italian and Japanese heritage. Her father died in an accident during her childhood, prompting her mother to return to Italy and leave Sonya behind in Japan. She was then adopted by Mai Kiryū, a nun and the matron of the academy's girls' dormitory, a past which contributes to her reluctance to discuss her upbringing.

Skilled at sewing, she creates paired stuffed animals—one representing a mother and the other a daughter—which she uses to perform shows for young orphans, revealing a compassionate and charitable side. She also works part-time as waitstaff at the school cafeteria, Dawn Hall, alongside her classmate Umi Hayasaka.

Her relationship with Umi is especially meaningful; the two have become inseparable friends. This bond marks a significant change in her social life, as she had few friends before coming to the academy.