Heidi's Grandfather is a central character known in the village by the names Alm-Onji, Alm-Öhi, or the Alm-Uncle. He lives a reclusive and solitary life in a mountain hut high above the Swiss village of Dörfli, with only his St. Bernard dog, Josef, for company. His seclusion is largely self-imposed, fueled by a reputation for being a bitter, godless, and ill-tempered old man. The villagers of Dörfli fear and avoid him, largely due to persistent rumors that he killed a man in his youth. This tragic event, along with the grief of outliving his son Tobias and daughter-in-law Adelheid (Heidi's parents), has led him to distrust the outside world and renounce any connection to the village or the church he once attended.

His personality at the beginning of the story is defined by this profound misanthropy and rugged independence. He is gruff, harsh, and initially unwilling to take on the responsibility of caring for his orphaned granddaughter, Heidi, when the girl’s aunt, Dete, abandons her at his doorstep. However, this stern exterior masks a deep well of unexpressed emotion. Heidi’s innate enthusiasm, curiosity, and unconditional affection slowly dismantle the emotional barriers he has built around himself. She is not frightened by his gruff demeanor, and her joy in the simple pleasures of mountain life—the goats, the flowers, the fresh air—awakens a gentleness and capacity for profound love he thought he had lost.

As a caregiver, he is practical and protective. Despite his own preference for isolation, he prioritizes Heidi’s well-being, as seen when he abandons a plan to sell a newborn goat after witnessing her distress over the separation. He teaches her the traditional skills necessary for mountain life, such as animal husbandry and cheese-making, and sustains them both through his own labor as a skilled woodworker, crafting bowls and utensils to trade with the villagers for essentials. His protectiveness becomes fiercely evident when Dete returns, now sporting fine clothes, to take Heidi away to Frankfurt to be a companion for a wealthy, wheelchair-bound girl named Clara Sesemann. He vehemently opposes this, but is ultimately tricked and powerless to prevent Heidi’s departure. Her absence devastates him; he retreats into an even deeper silence, sitting in despair and waiting for her return.

Heidi’s return, prompted by severe homesickness, marks a turning point. While his core identity remains rooted in the Alpine landscape, he begins a significant transformation, driven by his granddaughter's needs and newfound abilities. Recognizing that Heidi has learned to read in Frankfurt, he understands the importance of her education. In a monumental shift away from his former isolationism, he moves with her to a restored house in the village of Dörfli for the winter so she can attend school, and he even supports Peter, the young goatherd, in learning carpentry skills.

His role in the story is also pivotal to the recovery of Clara Sesemann. When Clara visits the Alps on a doctor’s recommendation, he accepts responsibility for her care. Using his extensive knowledge of the mountain environment, he actively participates in her physical therapy, carrying her to and from her wheelchair and devising gentle exercises using the terrain. With patient encouragement, he helps Clara take her first steps, culminating in her ability to walk unaided. Through his actions and Heidi’s influence, he gradually reintegrates into the community. He attends church again and befriends the village doctor, his journey concluding with a transformation from a feared and reclusive hermit into a respected and beloved member of Dörfli. Beyond his emotional evolution, he possesses notable practical abilities: he is a master carpenter, an experienced goatherd and cheese-maker, and possesses an encyclopedic knowledge of the mountain terrain, its flora and fauna, allowing him to live completely self-sufficiently.