Who is Cesare Borgia, Duc de Valentinois from Mirror Mirror and what is their importance?

Cesare Borgia, Duc de Valentinois, is Lucrezia Borgia's brother. He is the son of Vannozza Cattanei and Rodrigo Borgia, who becomes Pope Alexander VI. As a result of he and his sister's incestuous relationship, Cesare fathers Michelotto the gooseboy. Cesare is known for his good looks and unparalleled prowess in battle.