Nanni Spannu is an older man living in the Sardinian coastal village where Tottoi and his family settle after returning from Germany. A former environmental activist, he now lives as a reclusive figure, deeply connected to the natural world and the hidden marine life of the island. He is an old friend of Tottoi’s father, Cipriano, who trusts him enough to bring Tottoi to him after the boy claims to have seen a Mediterranean monk seal, a species long thought extinct. Spannu initially treats the claim with seriousness, agreeing to accompany Tottoi to the sea cave to investigate. When they confirm the presence of a mother seal and her pup, Spannu becomes the boy's cautious and protective advisor. He strongly urges Tottoi to keep the discovery secret, warning that revealing it would endanger the animals. His guidance is rooted in a deep respect for wildlife and a belief that human interference, even well-intentioned, can have harmful consequences. Despite his warnings, the secret eventually spreads, and when a wealthy American businessman arrives with the intent to capture the seals for an aquarium, Spannu joins the effort to stop him. He works alongside Captain Marco to intercept the poachers and ensure the seals are freed and relocated to safety. Throughout the story, Spannu remains a steady, morally grounded presence who prioritizes the welfare of the seals over any personal gain. His relationship with Tottoi is one of mentorship and trust, though he can be stern when he feels the boy has acted carelessly. He does not undergo significant change himself, but his actions are pivotal in guiding Tottoi’s understanding of conservation and responsibility.