Ermengarde St. John is a student at Miss Minchin's Select Seminary for Young Girls and becomes one of Sara Crewe's first and most loyal friends. She comes from an intellectual family; her father is a highly learned man who knows several languages and owns a vast library. However, Ermengarde does not share his academic gifts, a fact that causes her father great disappointment and puts significant pressure on her to perform well in her studies. This background has shaped much of her insecure and self-deprecating nature, as she has long been made to feel that she is not intelligent enough.

In terms of personality, Ermengarde is fundamentally kind, loyal, and good-hearted, though she is also deeply insecure about her lack of intellectual ability. She is frequently the target of ridicule from her classmates and even from Miss Minchin herself, who openly regards her as the class dunce. This treatment has left her earnest but struggling, often unhappy and lonely within the school environment. Despite her difficulty with rote learning and academic subjects like French, she possesses a quiet emotional depth and a fierce devotion to those who show her kindness. Her primary motivation is to find acceptance and friendship, as well as to live up to the expectations placed upon her by her family, even when those expectations seem impossible to meet.

Within the story, Ermengarde serves as one of Sara's key anchors during both prosperous and difficult times. When Sara first arrives at the seminary as a wealthy and admired student, Sara notices Ermengarde's unhappiness and reaches out to her, not with pity but with genuine sympathy and imagination. Sara draws Ermengarde into her rich imaginative world, telling her stories and treating her doll Emily as if she were alive, which deeply impresses Ermengarde, who is awed by Sara's creativity and intelligence. The two agree to become best friends. This friendship proves to be crucial and is tested severely when Sara loses her fortune and is reduced to the position of a scullery maid, forced to live in a cold attic.

In this later phase of the story, Ermengarde's role becomes even more significant. While many of the other girls distance themselves from Sara due to Miss Minchin's orders or their own discomfort, Ermengarde remains stubbornly loyal. She risks punishment by sneaking up to Sara's bleak attic room to visit her, continuing to seek her company and offer her support. These visits are a source of great comfort to Sara, helping to rekindle her temporarily dampened imagination and providing a connection to the warmth and kindness of her former life. Ermengarde's unwavering presence demonstrates that her friendship was never based on Sara's wealth or status, but on a genuine emotional bond.

Regarding key relationships, Ermengarde's connection with Sara is the central one of her life at the school. Sara treats her with a patience and respect that few others do, using storytelling and imaginative scenarios to help Ermengarde feel valued. Their friendship is reciprocal; while Sara offers intellectual and emotional guidance, Ermengarde offers steadfast devotion and a reminder that not everyone has abandoned Sara to her cruel fate. Ermengarde also has a notable relationship with the much younger student Lottie, as the two of them, along with the servant Becky, form a small circle of loyal companions around Sara during her time of hardship.

In terms of development, Ermengarde grows more confident through her friendship with Sara. Initially defined by her academic failures and the mockery of others, she gradually learns to value her own qualities of loyalty and kindness. Her courage to defy Miss Minchin and visit Sara in the attic marks a significant step in her personal growth, showing that she is capable of standing up for what she believes is right, even when it is difficult or frightening. Her character arc moves from a passive, unhappy girl who accepts the labels placed upon her to a more active and brave young friend who understands that her worth is not determined solely by her test scores.

Notable abilities or traits of Ermengarde are not academic in nature. She struggles significantly with memorization and traditional learning, which is why she is labeled the class dunce. However, her primary abilities lie in her emotional intelligence and her capacity for deep, unwavering loyalty. She shows a great deal of courage in maintaining her friendship with Sara despite social pressure and the risk of punishment. Her true strength is her heart, which is generous, forgiving, and constant. As Sara herself observes, the trials of adversity prove how truly nice and good Ermengarde is at her core.