Matty, sometimes called César, is Sebastian's grandfather and primary guardian. He lives with Sebastian and Angelina in a Pyrenees mountain village bordering France and Spain. Matty initially shares the community's deep suspicion of Belle, the large Pyrenean Mountain Dog, believing her responsible for livestock attacks and aligning with the villagers' efforts to capture or eliminate the perceived threat.

He exhibits strong protectiveness and care for Sebastian, especially regarding the boy's quest to find his Roma mother in Spain. This concern extends to Sebastian's bond with Belle, fearing the danger posed by hostile villagers and pursuing authorities. Matty's perspective evolves significantly. He gradually shifts from viewing Belle as a menace to acknowledging her loyalty, gentle nature, and unwavering devotion to Sebastian.

Ultimately, Matty champions Belle's cause, defending her against village accusations and hostility. He plays a crucial role in the resolution, supporting Sebastian's efforts to prove Belle's innocence and helping secure the community's acceptance. This transformation highlights his unwavering commitment to Sebastian's safety and emotional well-being, cementing his role as a steadfast guardian.