Mattia is a street-smart and resourceful boy whom Remi meets on the streets of Paris. He was born in Italy but has no memory of his birthplace and cannot speak Italian. His mother is deceased, and he was left to survive on his own, becoming part of a group of homeless children who live under the control of an exploitative caretaker. He initially makes a living through pickpocketing, but after meeting Remi and being influenced by her kindness, he gives up stealing and turns to honest work, such as shining shoes and performing music.

In personality, Mattia is mischievous, clever, and a little rowdy, but he has a deeply loyal and caring nature. He is practical and streetwise, understanding how to navigate the dangers of city life. Despite his rough upbringing, he is not hardened; he responds strongly to genuine kindness and becomes fiercely protective of those he cares about. His primary motivation is to stay by Remi's side, and he often speaks of wanting to work hard so that he can become worthy of that friendship. In the later part of the story, his ambition grows into a desire to achieve a stable and respectable life through his musical talent.

Mattia's role in the story is that of Remi's closest companion and traveling partner. After Remi leaves the Acquin family, Mattia joins him on the road, and the two form a small performing troupe together. Mattia provides both practical support and emotional grounding, often lightening difficult moments with his playful attitude. He also plays a key part in helping Remi during perilous situations, including orchestrating a rescue when Remi is wrongly caught up in a criminal scheme.

His most important relationship is with Remi, whom he admires for her gentleness and strong spirit. The two develop a deep, enduring bond, and Mattia promises to stay with her forever. He also forms a connection with the Milligan family later in the story. Mrs. Milligan ultimately adopts Mattia, making him Remi's adoptive brother, though he remains humble and driven by his own goals rather than relying on wealth.

Mattia's development is one of the clearest arcs in the story. He begins as a neglected street child who resorts to theft to survive, but through Remi's influence and his own growing self-respect, he transforms into an honest, hardworking, and ambitious young man. His talent for music is the vehicle for this change. When a music teacher encounters him, it becomes clear that Mattia is a musical genius with a natural gift for playing several instruments, most notably the violin. This discovery sets him on a path toward a successful career in music. By the end of the story, Mattia has become a renowned violinist, realizing the promise of the talent that was hidden beneath his difficult circumstances.