Aunt Dette, also known as Dete, is a secondary character in the story and serves as the maternal aunt and initial guardian of the young protagonist, Heidi. After the death of Heidi's parents when the child was an infant, Dette took on the responsibility of raising her niece. At the beginning of the narrative, when Heidi is approximately five years old, Dette makes the pivotal decision to bring the girl to live with her paternal grandfather, a reclusive man known as Alm-Onji, in the Swiss Alps near the village of Dörfli.

Dette's primary motivation is practicality, driven by her own circumstances and a belief in material advancement. She has secured a new job as a maid in the distant city of Frankfurt, a position she cannot take if she remains Heidi's caretaker. Convinced that her grandfather should assume responsibility for the child, she leaves Heidi with him, despite the villagers' concerns about his bitter and misanthropic reputation. While she appears to possess some degree of affection for her niece, her actions consistently prioritize her own financial stability and social positioning over Heidi's emotional well-being or desires.

Three years after leaving Heidi in the Alps, Dette returns with another opportunity that she believes is advantageous. She has learned of a wealthy German businessman, Mr. Sesemann, who is seeking a companion for his daughter Clara, who uses a wheelchair. Dette views this as a chance for Heidi to have a better life, particularly as the grandfather has refused to send the child to school or church. When Alm-Onji refuses to let Heidi go, Dette resorts to deception. She tricks the trusting girl into accompanying her under the false pretense of fetching presents for her grandfather and her friend Peter, the goatherd. During the journey to Frankfurt, Heidi realizes she has been misled and becomes deeply distressed, crying and protesting, but Dette remains unyielding. Upon arriving at the Sesemann household, Dette formally transfers guardianship of Heidi to the strict governess, Miss Rottenmeier, and promptly departs to pursue her own work, effectively abandoning the child in a strange and unwelcoming environment.

In her role in the story, Dette functions as a catalyst who sets the main plot in motion. Her first action of bringing Heidi to the mountains allows the central relationship between the girl and her grandfather to develop. Her second action of deceitfully taking Heidi to Frankfurt introduces the story's major conflict, leading to Heidi's intense homesickness and subsequent personal growth, including learning to read. Dette's relationship with Alm-Onji is notably antagonistic; she disregards his wishes and authority as Heidi's grandfather, and he in turn is deeply angered by her deception. Her relationship with Heidi is complex, defined by a mix of familial obligation and a fundamental inability to understand the child's deep connection to her mountain home and grandfather. Dette appears to genuinely believe that the opportunities in Frankfurt, such as formal education and material comforts, are superior to a life in the Alps, revealing a worldview that values urban prosperity over emotional and spiritual well-being.

Aunt Dette does not undergo any significant character development or redemption throughout the story. Her personality remains consistent: pragmatic, determined, and self-interested. She is approximately twenty-six years old during the events in Frankfurt. After delivering Heidi to the Sesemann residence, her role in the narrative is minimal, and she does not reappear. The story does not ascribe any notable abilities to her; she is depicted as an ordinary young woman driven by the practical concerns of securing her own livelihood. Her lasting legacy in the story is the unintentional consequence of her actions, which, despite her motives, ultimately lead Heidi to a life where she can flourish and bring joy and healing to those around her.