When Dete takes Heidi to live with her grandfather on the mountain, the misanthropic old man is initially reluctant to take care of the child. However, her intelligence and good nature quickly charm him, along with the whole community. By the time Dete is ready to take Heidi to Frankfurt three years later, her grandfather has grown to love her and does not want to let her go. His feelings are shared by Peter, his grandmother, and many others in the village, all of whom have come to love Heidi.

These feelings are reciprocated. Heidi finds much in Frankfurt that is new and exciting and has a good friend in Clara. However, she begins to pine for the mountains and her grandfather and has to return to avoid serious illness. Heidi's suffering at this point parallels that of her grandfather, and their growth through sadness reinforces the religious message of the book, as Clara's grandmother teaches Heidi to trust in God and Heidi performs the same function for her grandfather.

At the same time as the grandfather's sadness furthers the religious message of the story, it is also a testament to the particular magnetism of Heidi's personality, which drew him to her so quickly in the first place and has a similar effect on many others, including other children, such as Peter and Clara.