The character known as Billia is the cousin and first friend of the protagonist, Antonio, who is nicknamed Tottoi. Billia lives in Sardinia with his mother, who is the sister of Tottoi's father, Cipriano. When Tottoi and his family return to the island from Germany following the death of his mother, Billia is among the first to welcome them. He immediately befriends his cousin and gives him the local nickname Tottoi without asking for permission.

Billia is a local boy who is familiar with the island's traditions, geography, and people. His personality is characterized by a mix of friendly openness and healthy skepticism. He is welcoming and quickly bonds with Tottoi, acting as a companion and guide to his new surroundings. Despite their close friendship, Billia does not blindly believe everything Tottoi says. When Tottoi excitedly claims to have seen a Mediterranean monk seal in a local sea cave, an animal believed to have been extinct in the area for centuries, Billia is not convinced and doubts the story. This reaction establishes him as a grounded and realistic individual, in contrast to Tottoi's more wonder-struck and determined nature.

Billia works for Captain Marco, a local boat operator who takes tourists to the famous sea caves, including the one where the seals are later discovered. His role in the story is primarily that of a supportive peer. He represents the local youth and, along with other friends like Caterina and Pedro, becomes involved in Tottoi's secret. Although initially skeptical, Billia's loyalty to his cousin leads him to join the group of teenagers who go to see the seals for themselves. In these interactions, Billia and the other young people are described as being very gentle with the animals. This event demonstrates his curiosity and his willingness to trust his cousin's experiences once presented with evidence. Through this involvement, Billia transitions from a disbelieving local to an accomplice in protecting the secret of the seals' existence.

While Billia does not possess any special abilities, his notable trait is his practical knowledge of the local environment, gained from living in Sardinia and working on the tourist boats. This knowledge makes him a valuable and reliable companion for Tottoi as the story unfolds. His relationship with Tottoi is the central one for his character, defined by a natural and uncomplicated cousinly bond that helps anchor the protagonist to his new home and community.