In the anime A Little Princess Sara, Marguerite, known in the original Japanese version as Ermengarde St. John, is a student at Miss Minchin's Select Seminary for Young Ladies and becomes one of Sara Crewe's closest and most loyal friends. She is portrayed as a plump, good-hearted girl who struggles significantly with her studies, often finding herself at the bottom of the class despite her sincere efforts to learn. Facts, numbers, and foreign languages, particularly French, are difficult for her to grasp, and this academic weakness makes her a frequent target of ridicule from other students, especially the class representative Lavinia Herbert. Her background is that of a boarder at the seminary, where she is accustomed to being overlooked and laughed at for her poor performance, leaving her with low self-esteem and a deep sense of loneliness.

Marguerite's personality is defined by her kindness, gentleness, and unwavering loyalty. She is not clever in the traditional academic sense, but she possesses a pure and compassionate heart. Her primary motivation is the desire for friendship and acceptance, which is why Sara's decision to befriend her means so much. When Sara defends her from teasing and shows her kindness despite her academic failures, Marguerite is deeply moved and grateful. This bond becomes a cornerstone of her character, and she remains steadfastly devoted to Sara even after Sara loses her wealth and is reduced to a servant. While other students distance themselves or turn cruel, Marguerite continues to visit Sara in the attic, risking punishment to maintain their friendship and offer comfort. She is frequently described as slow but loving, and her loyalty never wavers throughout the story.

In the narrative, Marguerite serves as Sara's first true friend at the school and represents the theme that genuine companionship is not based on wealth or intelligence. Her role is primarily supportive, providing emotional warmth and a safe connection for Sara during both her prosperous and impoverished days. Her key relationships include her deep friendship with Sara, a warm but often protected bond with the younger student Lottie, and a tense dynamic with Lavinia, who sees her as an easy target. Over the course of the series, Marguerite undergoes modest development. She gains confidence through Sara's encouragement and learns to value her own worth beyond her academic shortcomings. Her most notable abilities are her capacity for deep compassion and her brave determination to stand by those she loves, even when doing so is difficult or dangerous. She does not possess special skills, but her emotional strength and faithfulness make her an indispensable pillar of support for Sara.