Sonja was the teenaged daughter of Vinton Harper and his first wife, Mitzi. In typical teenage fashion, she often appears bored. She was older, but she cafed when she realized that she was often overlooked by younger and more readily obedient brother, Vinton "Buzz" Harper, Jr.
Also, like her aunts, Ellen Jackson and Eunice Higgins, she also had some resentment towards Buzz. He always seemed to do the right thing, while she always messed up. She was older and got an earlier curfew, and he didn't.
She mostly acts like she was bored, and is considered a slacker, but she clearly does love her family. She often drives her grandmother, Thelma Harper crazy, much like her aunts did, when they were her age.
Since she shared a bathroom with Buzz and her great-aunt, Fran Crowley, this led to more arguments. Sonja was quite sloppy, unlike neat and orderly Buzz (another sore spot) and this made the already nervous Fran even more nervous. Later on, however, Sonja adopted a classier look and style to her appearance and clothing.
Sonja left Raytown, along with her brother, after two years to parts unknown. She, like her brother, formed a bond with her father's second wife, Naomi Oates Harper.