Peter is an eleven-year-old goatherd tasked with leading the villagers' goats to pasture in the Swiss Alps. He resides in a modest shack near Dörfli with his mother, Brigitte, and his blind grandmother, enduring financial hardship and hunger before Heidi's arrival.

Initially sullen, pessimistic, and indifferent to school, Peter avoids lessons and struggles with literacy. He harbors jealousy over Heidi's friendships, particularly with Clara Sesemann, a wealthy disabled girl from Frankfurt. This jealousy culminates in an incident damaging Clara's wheelchair—accidentally in the anime adaptation due to Clara's hesitation about walking.

Heidi's presence significantly alters Peter's path, alleviating his loneliness. Witnessing her literacy, especially when she reads hymns to his grandmother, fulfilling a long-held wish, sparks Peter's own appreciation for learning. He discovers a natural aptitude for carpentry, building his own sled and winning a local race—an achievement that bolsters his confidence and social standing.

Peter's involvement in Clara's physical therapy marks substantial growth. Moving from initial resentment, he actively contributes to her recovery by guiding her through the mountain terrain and assisting with exercises, reflecting a shift toward supportive friendship. By the series' conclusion, Peter is approximately sixteen years old, demonstrating increased maturity and emotional resilience. His bond with his grandmother remains central, with Heidi's kindness towards her underscoring both Peter's initial limitations and his eventual empathetic development.