The character known as Abuelo, or the Old Man of the Alps, is a central figure in the alpine landscape where the story takes place. He is a man of few words, marked by a past shrouded in rumor and personal tragedy. The villagers of Dörfli refer to him by the nickname Alm-Onji, meaning Alps-Uncle, a title that reflects both his long residence in the mountains and the community's distant, wary respect for him. Physically, he is depicted as a tall and strong elderly man, possessing a large white beard and thick eyebrows, a visage that mirrors the rugged, untamed nature of his surroundings.

His background is one of significant loss and self-imposed exile. In his youth, he was the heir to a prosperous farm in the Domleschg region of the Swiss Grisons, but he squandered his inheritance through gambling and drinking, a period of his life that led to his parents dying of grief. After disappearing for many years, he returned to the area with a teenage son named Tobias. He settled in the village of Dörfli and took up an honest trade as a carpenter. However, tragedy struck again when his son, now an adult and a father, was killed in a workplace accident when a beam fell on him. Shortly after, his daughter-in-law Adelheid died of grief, leaving their young daughter, Heidi, an orphan. Consumed by sorrow and a profound depression, the old man retreated from village life entirely, isolating himself in a hut high up on the alp, accompanied only by his loyal Saint Bernard dog, Josef.

At the start of the narrative, his personality is defined by bitterness, silence, and a deep distrust of others. The villagers fear him, perpetuating dark rumors that he was once involved in a violent conflict that resulted in a man's death. He is perceived as godless, ill-tempered, and harsh, a recluse who has severed all ties with the community below. His motivations are simple: to be left alone to live out his remaining years in solitude. This all changes with the sudden arrival of his five-year-old granddaughter, Heidi, whom her aunt Dete abandons at his doorstep. His initial motivation is to reject this responsibility, but Heidi's innate curiosity, joy, and unconditional affection slowly dismantle the walls he has built around his heart.

His role in the story is multifaceted. He is first and foremost Heidi's guardian and protector, providing her with a simple but secure life in the mountains. He becomes a surrogate parent who teaches her the ways of the alpine world. As the narrative progresses, his role expands to that of a healer and a facilitator of miracles. When Heidi is taken against her will to Frankfurt, the old man is left devastated, returning to his solitary ways while waiting for her return. Upon her homecoming, his transformation continues. Seeing that Heidi has learned to read, he decides to restore a dilapidated house in the village of Dörfli, so that she can attend school during the harsh winters. This act marks his first real step toward reintegrating into the community he had rejected for so long.

His key relationships are the cornerstone of his character development. The most significant is, of course, his bond with Heidi. She is the catalyst for his redemption, bringing light and purpose back into his existence. He also develops a crucial relationship with Clara Sesemann, the wealthy, wheelchair-bound girl who comes to stay in the Alps on the advice of her doctor. The old man initially agrees to care for her, and using his profound knowledge of the mountains and a firm, kind guidance, he actively participates in her physical therapy alongside Heidi and the goatherd Peter. His efforts are rewarded when Clara is finally able to stand and walk unaided. This success helps to fully heal his reputation in the eyes of the villagers, who begin to see him not as a frightening hermit, but as a wise and capable elder.

Throughout the story, Abuelo demonstrates remarkable abilities. He is an expert carpenter and woodworker, crafting bowls, utensils, and even furniture, which he trades for the necessities of life like bread and cheese. He is a skilled goatherd and cheesemaker, entirely self-sufficient in his alpine home. His most profound ability, however, is his deep, intimate knowledge of the mountain environment, its weather, its plants, and its animals, a knowledge that ensures his and Heidi's survival and well-being. His development is a powerful arc of transformation, moving from a reclusive, bitter, and feared figure to a respected, loving, and integral member of the alpine community, all brought about by the simple, unwavering love of his granddaughter.