Son of a legendary assassin, Chiaro was ordered to kill Cesare in order to set his father free.
In the end, he actually saved Cesare's life and grew to become his best friend.
During Cesare's days of tyrany, Chiaro served under Cesare; killing anyone that dared to oppose any of his best friends plans or dared to get in his way.
There is speculation that Cesare had improper feelings and affection for Chiaro (as any one could expect from a work of Higuri You's...), but Chiaro fell for Cesare's younger sister, Lucrezia.
These feelings for her cause much strife and problems throughout the series.