In Assassin's Creed II, Ezio's uncle who runs the villa at Monteriggioni is Mario Auditore.
Mario is the older brother of Giovanni Auditore and a veteran Assassin who lives at Villa Auditore in Tuscany. After Ezio's family is executed in Florence, Mario rescues Ezio and his younger sister and mother, then introduces Ezio to the Assassin way of life.
The villa becomes a major base during the game. Ezio can improve its buildings, increase its income, and recruit allies there. Mario also helps Ezio during important missions, including the defense of Monteriggioni.
A common mix-up is confusing Mario with Giovanni, Ezio's father. Giovanni is the Florence-based Assassin whose secret work drives the opening plot, while Mario is the uncle who protects the family and controls the countryside stronghold. Monteriggioni itself is a real medieval hill town in Tuscany, although the game expands its historical role for the story.