Who was Cesare Borgia in The Prince?
I edited your question since you can only ask one question at a time.
Cesare Borgia was a member of a notorious Renaissance family led by Alexandre Borgia, who was a Pope. He was not your average Pope: All Borgias were the medieval "Sopranos", always bribing, committing some form of crime, and obtaining riches, goods and loads of power in the darkest places.
Cesar was the eldest of the Borgia son, and brother to another notorious lady of history: Lucrezia Borgia.
Cesar was a young, horrid and ambitious warrior that stopped at nothing to become the celebrated "Prince" of which Niccolo Machiavelli speaks of: One who does not need divine kingship to be elected, and one who does not need to be fair and just: Just bad and wild and strong enough to grab a throne away from anybody.
He was conqueror of many places of which he became Duke, and with the godfather-hood of the Pope he was elevated to many positions of power whenever the Borgias wanted more.
He was Dule of Romagna and Valentinios, and Prince of Andria and Venatro, among many other titles acquired either by force, by his father's influence, or by marriage. He was only 31 when he died.