Apollonia was the Sicilian princess Michael Corleone found while hiking with his bodyguards Fabrizio and Calo in the hills of Corleone, Sicily. You could tell how much Michael loved Apollonia Vitelli- Corleone by the way Michael explained her murder to his son and daughter after Anthony sang that Sicilian song to him. The song clearly reminded him of her. His bodyguard Fabrizio was the guy behind her murder. The other bodyguard Calo became the leader of the Sicilian crew of the Corleone family. I think Calo became the caporegime after Don Tommosino was killed in GF3. I also think Apollonia would have been a lot better for Michael than Kay ever was. Apollonia was his true love.