Who is Biagio Piovasco di Rondo from The Baron in the Trees and what is their importance?

Biagio is Cosimo's younger brother and the narrator of the story. He is, in the boys' early youth, Cosimo's partner in adventure and meting out of justice. When the two of them are so horrified by their sister's cooking small animals and their parents' subsequent insistence that the family share her lavishly executed animals at family meals, they conspire together a means of setting her next victims free. Biagio understands his brother's feeling of abandonment when he enters the trees, and his heart is divided between his family and loyalty to his brother and best friend. As they grow, Biagio is both an admirer and a confidante to Cosimo, and for that reason is able to faithfully recount the story of Cosimo's life both as he observes it and as it is recounted to him in the time he spends listening to Cosimo's stories.