Cesare Borgia serves as the principal antagonist of Assassin's Creed Brotherhood.
The game continues Ezio Auditore's story after Assassin's Creed II and is set primarily in Rome during the early 16th century. Cesare Borgia, a military commander and son of Pope Alexander VI, leads the opposing Borgia forces. In the game's fictionalized historical narrative, Ezio works to weaken Cesare's power and rebuild the Assassin Brotherhood in Italy.
Cesare was a real Renaissance figure whose career was closely connected to the Borgia family's political ambitions. The game uses real names and events as a foundation, but its supernatural elements, secret societies, and direct personal confrontations are fictionalized for the story.
Rodrigo Borgia, Cesare's father, is the main antagonist of Assassin's Creed II rather than Brotherhood. That family connection is a common source of confusion. Brotherhood also introduced a larger focus on recruiting and directing fellow Assassins, alongside its Rome-based open world.