Mr. Rodrigo Borgia a.k.a. Pope Alexander VI. He mad many alliances to secure his position among these are with the French and a bunch of people from the House of Sforza, this alliance however wouldn't last for long because of his greed. He viewed Girolamo Savonarola as a hostile to his position. Due to his actions, the Houses of Colonna and Orsini allied against him.
Cesare Borgia is Rodrigo's son and fought for conquest of Italy under his father's "guidance". He attacked several regions of one of these is Forli. In this battle, Carlo Orsini and Bartolomeo d' Alviano, who were allied together, defeated Cesare. Cesare allegedly hired Leonardo da Vinci and became "intimate". Cesare was also "admired" by Niccolò Machiavelli, however, upon closer inspection, actually "admired" Mr. Rodrigo.
The Duke of Milan, Galeazzo Sforza of the House of Sforza was assassinated. This assassination was likely influential to the Pazzi Conspiracy. He had an illegitimate daughter, Caterina Sforza.
Girolamo Riario was the husband of Caterina Sforza. He was said to have been one of the plotters of the Pazzi Conspiracy, though he is not viewed as this in the game, he is an enemy of Lorenzo de' Medici. After the failure of the conspiracy, he was the last of the plotters alive. He was assassinated by the Orsi Brothers, who were hired by Caterina, his own wife. These events are "replayed" with the Battle of Forli. He had 5 sons and a daughter, of these, the notable are Ottaviano Riario and Bianca Riario.
Caterina Sforza wife of Girolamo Riario. She had him killed by the Orsi Brothers, who in turn, supposedly then worked for Mr. Rodrigo to take Caterina and her children as prisoners. With the assistance of Ludovico Sforza, she regained control of Forli. Once Mr. Rodrigo became Pope, it was supposedly in her favor as ruler; he was even the godfather of Ottaviano Riario. Soon, however, after a lot of disastrous events, Caterina was against Cesare Borgia, and ultimately, Mr. Rodrigo. She lost that battle and was held "guest". To justify the imprisonment of Caterina, Mr. Rodrigo accused her of trying to kill him with letters impregnated with poison, Mr. Machiavelli believed it. She was later on released and she went to live in Florence.
Ludovico Sforza, brother of Galeazzo Sforza, uncle of Caterina Sforza, ally of Mr. Rodrigo, patron of Leonardo da Vinci.
The Pazzi Conspiracy. Francesco Salviati, Girolamo Riario, Francesco de' Pazzi and Jacopo de' Pazzi. The plot to dethrone the Medici for control of Tuscany. The conspirators were hunted down throughout all of Italy. You know the rest of the story.
Lorenzo de' Medici, enemy of the Papacy, the Pazzi Conspirators. Patron and supporter of many scholars, poets and artists. Survivor of the Pazzi Conspiracy, in the aftermath, suffered from the wrath of the Vatican. However, his diplomacy saw him through. He took Poliziano as one of his friends. Lorenzo's court included artists, of these quite notable in this relativity is Leonardo da Vinci. He died at the beginning of the "Age of Exploration" with Christoffa Colombo a.k.a. Christopher Columbus at its center. His death also caused the Renaissance to shift from Florence to Rome.
Girolamo Savonarola. Enemy of Mr. Rodrigo, Ludovico Sforza. He was not openly an enemy of the Medici as he blessed Lorenzo de' Medici on his deathbed. He was, however, an enemy of the Renaissance causing the Bonfire of the Vanities. He was excommunicated and executed by Mr. Rodrigo because of him being a rising threat to his position. Mr. Machiavelli wrote about Savonarola's execution.