Sarah, a central figure in Sebastian's journey, resides in Spain as the affluent Mr. Albert's daughter. Despite her privileged life, isolation and illness foster deep loneliness and a longing for true friendship. Her ninth birthday celebration during the series confirms she is near Sebastian's age.

Upon meeting, Sarah and Sebastian form an instant, profound bond through their shared need for companionship. She reappears frequently in his travels, offering steadfast assistance. Her father's initial dislike of dogs softens as Belle and Poochie visibly improve Sarah's health and happiness.

An intense emotional connection develops between Sarah and Sebastian, hinting at future romance. The narrative implies their eventual marriage as adults, with Belle remaining a lifelong companion. Mr. Albert's growing admiration for Sebastian's mother, Isabel, suggests potential familial ties between their households. Sarah's kindness and pivotal support cement her role in Sebastian's quest for belonging.