Sarah Frances Russell, eldest daughter of George Russell, Viscount of Marble, and his tragically deceased first wife Lady Frances Russell, grew up without memories of her mother, knowing her only through inherited possessions: a flute and a musical score. She resides at Marble Mansion with her father and her younger half-sister, Lynn, who arrived from Japan after their mother's death in an accident. Sarah initially greets Lynn with coldness, masking her inherently kind and caring nature. Her artistic sensitivity manifests in skilled painting and flute playing.

Over time, sharing the profound experience of motherlessness, Sarah's feelings for Lynn transform. She sheds her initial reserve, developing a protective, almost maternal affection for her half-sister. This loyalty drives Sarah to staunchly defend Lynn against antagonism. Her relationship with Arthur Brighton, the elder son of a neighboring noble family, predates Lynn's arrival. Their longstanding friendship, strengthened by shared interests like equestrian pursuits and Arthur's calm demeanor complementing Sarah's nature, hints at deeper emotional ties.

The arrival of Magdalene Waverley and her children, Thomas and Mary, brings conflict. Magdalene schemes to marry George Russell while her children torment Lynn. Sarah becomes Lynn's steadfast protector against the Waverleys, showcasing her courage. Her development centers on embracing her role as Lynn's sister and guardian, evolving from an aloof figure into a source of emotional support and familial strength.