Charles V is the young King of Spain and (after his election) Holy Roman Emperor, giving him control of various minor territories in Italy, Austria, Germany and the Low Countries; he is easily the most powerful monarch in Europe. Charles also controls Spain's considerable, newly-established colonies in the Americas, which he mentions to Henry in episode 1.03, suggesting they should both seek fresh conquests in the New World.
He is the nephew of Henry VIII's Spanish first wife, Queen Catherine of Aragon, making him Henry's nephew by marriage; although brought up in Spain, he has the distinctive chin of the Austrian Habsburgs. Although Charles only appears physically in episode 1.03 of the series (he is usually represented by his Ambassador, Eustace Chapuys) he is mentioned in nearly every episode, referred to simply as 'the Emperor'. Despite not intervening much on the behalf of his aunt and his cousin Mary (as well as breaking his own betrothal to Mary, to marry a Portuguese Princess), Charles is genuinely concerned with their well-being, affecting his diplomacy with both Henry and the Papacy and in turn, provoking Henry to begin the Reformation so that he can divorce Catherine.
While Charles and Henry have been allies on several occasions and admire each other's power, they are also sometimes enemies. Henry is deeply resentful of the Emperor throughout the series, since Charles has a tendency to break their agreements prematurely if he thinks Henry will not follow through; neither of them is willing to give the other real trust, and therefore their alliances nearly always break down after a short time. Charles' real feud, however, is usually with King Francis I of France, and he and Francis are usually vying for an alliance with England, with Henry choosing whichever side suits him at the time. Charles is also occupied with countering the expansion of the Turkish-led Ottoman Empire and the Protestant League in Germany, both of whom King Francis later becomes allies with. Only at one point in Season 3 does Charles make peace with France long enough to pose a serious threat to England, and nothing comes of this.
Charles is played by Spanish-English actor Sebastian Arnesto.
Backstory of Charles V
Born in 1500 in the Low Countries, as heir to the Houses: Trastámara, Habsburg and Valois-Burgundy Charles became King of both Castile and Aragon in 1516 and Archduke of Austria on 1519, whereupon he was also elected Holy Roman Emperor. From his mother, "Juana the Mad", Charles inherited the Spanish kingdoms of Castile and Aragon (which together became the Kingdom of Spain under his successors) as well as the Kingdoms of Sicily-Naples and Sardinia in Italy and several blossoming colonies in the Americas. From his Habsburg grandfather Emperor Maximilian I, he inherited the Archduchy of Austria (and, by claim, the Kingdoms of Bohemia and Hungary) and from his grandmother, Mary of Burgundy he inherited the Low Countries (modern day Netherlands, Belgium and Luxembourg) . Through the Trastámara dynasty, he descended from Katherine of Lancaster, he like Catherine of Aragon, he had English blood. And through the House of Valois-Burgundy, he descended from Margaret of York, he had both Lancastrian and York blood. In 1521 he became engaged to his cousin Mary Tudor, Princess of Wales, but the marriage was delayed as Mary was only a child; by the time of Charles' victory over the French at the Battle of the Pavia in 1525, he no longer needed an alliance with England and abandoned the engagement, instead marrying Isabella of Portugal, with whom he had three children. Charles was a devoted husband and father, and unlike his French or English counterparts he never took a mistress during his marriage; when Isabella died in 1539, Charles was grief-stricken. He never remarried, and wore black for the rest of his life as a sign of mourning.
Charles was famous for developing a highly efficient and powerful army which he used to good effect against various rivals, mainly the Kingdom of France and the Ottoman Empire, the latter of which conquered most of Hungary before finally being stopped at the Siege of Vienna in 1529. However, while his wars against the French (fought mainly in Italy, North Africa and Germany) were mostly successful on land, his navy suffered various defeats against the Ottoman fleets. Charles' wars also put Spain deeply into debt, and Charles himself was a frequent advocate for peace despite being constantly at war. Charles also presided over the era of the Conqustadores, during which the Spanish forces lead by Hernán Cortés and Francisco Pizarro overthrew the Aztec and Inca empires, providing Spain with an immense source of riches from the Americas.
Due to his missionaries' successes in America and his enmity with the Turks, Charles became increasingly convinced of his status as the 'champion' of Catholicism. This was in spite of his sacking of Rome in 1527 during his wars with France, an event that inadvertently led to the thwarting of Henry VIII's annulment of his marriage, thus destroying Charles' alliance with England and making way for the English Reformation. With the growth of the Protestant Reformation in Central Europe, Charles faced increasing religious division at home and in the mostly-Lutheran Holy Roman Empire, the former leading him to approve of the Spanish Inquisition to suppress 'heresy' and the latter leading him to war in Germany from 1546-47. Although the Inquisition halted the spread of Lutheranism in Spain, and his armies defeated those of the German 'Protestant League', Charles failed to destroy Protestantism or armed resistance within the German States, and in 1555 he signed the Treaty of Augsburg with the German Princes, allowing them freedom to choose between Protestantism and Catholicism in return for acknowledging him as Holy Roman Emperor. Despite the brutal measures of the Inquisition and his various enemies abroad, Charles remained popular within Spain and Portugal for his peaceful and (relatively) just rule.
Charles kept both his Royal titles until voluntarily abdicating in 1556, whereupon his Austrian territories and position as Emperor were given to his brother Ferdinand and his Spanish Empire (including his Italian and Flemish territories) was inherited by his eldest son Phillip II; the two empires remained loosely allied for the next two centuries, as both were ruled by different branches of the House of Habsburg. Although Charles was only 56 when he abdicated, his 34-year reign and various health problems had exhausted him; he retired to a monastery, dying two years later of malaria in 1558.