Moustache, the primary gardener for Tistou's family estate, assumes the role of the boy's mentor when Tistou leaves formal schooling. He continues the child's education through practical experience in the garden, emphasizing gentleness, patience, and direct interaction with nature, offering a distinct contrast to the strictness of Tistou's father and other authorities.

Affectionately known as "Mr. Mustache," he guides Tistou in understanding the garden's biodiversity and the interconnectedness of living things. Under his guidance, he discovers and cultivates Tistou's extraordinary "green thumbs" power—the ability to make plants bloom instantly upon touch—encouraging the boy to use this gift to spread beauty and joy.

Beyond horticulture, he instills values of kindness and empathy, particularly towards marginalized members of society. He actively supports Tistou's altruistic efforts to beautify the town's prison, hospital, and impoverished areas. His influence profoundly shapes Tistou's worldview and commitment to improving his community.

His later death deeply impacts Tistou. In response, Tistou creates a floral ladder ascending to the sky, symbolically seeking reunion. This act underscores their deep emotional bond and cements Moustache as a lasting symbol of compassion and harmony with nature throughout the story.