Cardinal Mazarin is the regent of Tristain at the start of the series, when the king died and the queen is too distraught by the king's death to rule. Princess Henrietta at the time was planned to be married in a political marriage to Germania to cement a political alliance.
In the novels, he is far from the nobles who oppress the commoners and serves the royalty of Tristain with absolute loyalty. He was tipped to be the next in line for the throne of Romalia's Holy Emperor and Pope, but declined in order to serve the Tristain royal family and this lead to Vittorio acceding to the Romalian throne. His motive for doing so is unclear, but fears of him seizing the throne of Tristain ultimately proved to be unfounded as Henrietta took over.
His loyalty is more to the Tristian's royal family than the ruling monarch, as demonstrated when both he and Henrietta's mother pushed for Henrietta to marry a suitable noble in order to stabilize the country's political situation after King Joseph's death.
Mazarin is named after Cardinal Jules Mazarin (1602 – 1661), born Giulio Raimondo Mazzarino, a French politician.