Fratello is the mongrel dog of Sandro, who found him and named him Fratello, the Talian word for "brother". He is later renamed "Brother Dog" when he joins Sandro at the friary at Saint-Mary-among-the-Vines.
Fratello initially appears as a mongrel dog that Davide Nucci stops to pet during an evening stroll with his older brothers, Camillo and Filippo. However, Davide is soon stabbed by Carlo di Chimici under the guidance of Sandro's master, the di Chimici spymaster Enrico Poggi. A frightened Sandro witnesses the entire event and begins avoiding Enrico. He winds up adopting the mongrel dog that Davide had been petting; though Sandro would like to name the dog after the murdered Nucci boy, he does not know Davide's name and decides to name the dog "Fratello" because the murdered boy had been someone's brother.
Fratello becomes Sandro's constant companion, accompanying the boy during Sandro's excursions throughout Giglia, including Brother Sulien's friary, though he is often kept out in the yard. During one of their excursions near the Piazzo Ducale, Sandro sees Carlo di Chimici and tries to leave, though Fratello recognizes Carlo and snarls at the di Chimici prince. Fratello's snarling is seen by Camillo Nucci, who recognizes the dog and realizes that Carlo had been responsible for Davide's death.
Both Sandro and Fratello eventually find a home at the friary at Saint-Mary-among-the-Vines when Sandro is accepted as a novice friar. Fratello is renamed "Brother Dog" and allowed to live in the kitchens with Brother Tullio to keep the rats out. They adjust well to their new life at the friary, apparently causing daily disruptions for Sulien, and enjoying the kitchens.